Worker Larvae Research Articles

Expression of insulin pathway genes during the period of caste determination in the honey bee,Apis mellifera

Female honeybees have two castes, queens and workers. Developmental fate is determined by larval diet. Coding sequences made available through the Honey Bee Genome Sequencing Consortium allow for a pathway-based approach to understanding caste determination. We examined the expression of several genes of the insulin signalling pathway, which is central to regulation of growth based on nutrition. We found one insulin-like peptide expressed at very high levels in queen but not worker larvae. Also, the gene for an insulin receptor was expressed at higher levels in queen larvae during the 2nd larval instar. These results demonstrate that the insulin pathway is a compelling candidate for pursing the relationship between diet and downstream signals involved in caste determination and differentiation.

Behavioural responses of workers towards worker‐produced male larvae and queen‐produced worker larvae in Acromyrmex subterraneus brunneus Forel, 1911 (Hym., Formicidae)

Abstract: Leaf‐cutting ant workers care for larvae with an elaborate behavioural repertoire to satisfy the needs of the offspring. In order to investigate worker discrimination ability in the leaf‐cutting ant Acromyrmex subterraneus brunneus, we compared the behavioural responses of workers towards worker‐produced male larvae and queen‐produced worker larvae, including the following behavioural acts: licking the larval body, transporting larvae, feeding larvae with hyphae, scraping larval mouth parts, ingesting faecal fluid excreted by the larva, and inserting hyphae of the symbiontic fungus on the larval body. We verified that workers behaved differently depending on larval origin, i.e. licking the larval body, transporting larvae and inserting hyphae of the symbiontic fungus on the larval body. Observed differences denote the ability of workers in discriminating among larvae, probably because of their different individual needs.

Semiochemicals Influencing the Host-finding Behaviour of Varroa Destructor

Studies of Varroa destructor orientation to honey bees were undertaken to isolate discrete chemical compounds that elicit host-finding activity. Petri dish bioassays were used to study cues that evoked invasion behaviour into simulated brood cells and a Y-tube olfactometer was used to evaluate varroa orientation to olfactory volatiles. In Petri dish bioassays, mites were highly attracted to live L5 worker larvae and to live and freshly freeze-killed nurse bees. Olfactometer bioassays indicated olfactory orientation to the same type of hosts, however mites were not attracted to the odour produced by live pollen foragers. The odour of forager hexane extracts also interfered with the ability of mites to localize and infest a restrained nurse bee host. Varroa mites oriented to the odour produced by newly emerged bees (<16 h old) when choosing against a clean airstream, however in choices between the odours of newly emerged workers and nurses, mites readily oriented to nurses when newly emerged workers were <3 h old. The odour produced by newly emerged workers 18-20 h of age was equally as attractive to mites as that of nurse bees, suggesting a changing profile of volatiles is produced as newly emerged workers age. Through fractionation and isolation of active components of nurse bee-derived solvent washes, two honey bee Nasonov pheromone components, geraniol and nerolic acid, were shown to confuse mite orientation. We suggest that V. destructor may detect relative concentrations of these compounds in order to discriminate between adult bee hosts, and preferentially parasitize nurse bees over older workers in honey bee colonies. The volatile profile of newly emerged worker bees also may serve as an initial stimulus for mites to disperse before being guided by allomonal cues produced by older workers to locate nurses. Fatty acid esters, previously identified as putative kairomones for varroa, proved to be inactive in both types of bioassays.

Quantifying honey bee mating range and isolation in semi-isolated valleys by DNA microsatellite paternity analysis

Honey bee males and queens mate in mid air and can fly many kilometres on their nuptial flights. The conservation of native honey bees, such as the European black bee (Apis mellifera mellifera), therefore, requires large isolated areas to prevent hybridisation with other subspecies, such as A. m. ligustica or A. m. carnica, which may have been introduced by beekeepers. This study used DNA microsatellite markers to determine the mating range of A. m. mellifera in two adjacent semi-isolated valleys (Edale and Hope Valley) in the Peak District National Park, England, in order to assess their suitability for native honey bee conservation and as isolated mating locations. Three apiaries were set up in each valley, each containing 12 colonies headed by a virgin queen and 2 queenright drone producing hives. The virgin queens were allowed to mate naturally with drones from the hives we had set up and with drones from hives owned by local beekeepers. After mating, samples of worker larvae were taken from the 41 queens that mated successfully and genotyped at 11 DNA microsatellite loci. Paternity analyses were then carried out to determine mating distances and isolation. An average of 10.2 fathers were detected among the 16 worker progeny. After correction for non-detection and non-sampling errors, the mean effective mating frequency of the test queens was estimated to be 17.2, which is a normal figure for honey bees. Ninety percent of the matings occurred within a distance of 7.5 km, and fifty percent within 2.5 km. The maximal mating distance recorded was 15 km. Queens and drones did occasionally mate across the borders between the two valleys, showing that the dividing mountain ridge Losehill does not provide complete isolation. Nevertheless, in the most isolated part of Edale sixty percent of all matings were to drones from Edale hives. The large majority of observed mating distances fell within the range of Hope Valley, making this site a suitable location for the long term conservation of a breeding population of black bees.

A CINDERELLA STORY

In human societies, not everyone is born equal. The phrase `born with a silver spoon in the mouth' refers to the fact that, at birth, some people find themselves in positions of privilege and wealth whereas others do not. Similar inequalities exist within animal societies and, in the case of some insects such as ants and bees, are taken to great extremes. Ants and bees have evolved eusociality – several generations live together in colonies, only one or a few individuals (queens) have offspring, and non-reproductive colony members, the workers, care for these offspring. Becoming a queen is not something that workers can aspire to. Although some sneaky workers do try to have offspring themselves, individuals born as workers cannot become queens. Nutrition is an important determinant of social class in many eusocial insect species. In some species, larvae destined to become queens are fed a particular substance (royal jelly in honey bees), whereas in other species queen larvae get more food than worker larvae. This latter system is found in the stingless bee Schwarziana quadripunctata. Now, an international team of researchers from Belgium, Britain and Brazil led by Tom Wenseleers has discovered that some stingless bees manage to beat the system and change their fate.The team investigated previous reports that dwarf queens occur in stingless bee colonies in addition to the normal, large queens. Wenseleers and his co-workers weighed the dwarf queens and showed that they are indistinguishable in weight from regular worker bees. In addition, dwarf queen larvae are raised in cells that are identical to the cells in which worker larvae are raised,which are smaller than the large specialized cells in which the queen larvae develop. Dwarf queens aren't a rare occurrence; when the team assessed the social class of 11 574 individual stingless bees from 19 colonies, they found six times more dwarf queens than normal queens. The team used a series of morphological measurements to identify adult dwarf queens and distinguish them from normal queens. They showed that the dwarf queens headed 12 out of a further 54 colonies, suggesting that the dwarf queens are able to reproduce. Although this may seem like a large proportion of colonies, it actually suggests that dwarf queens aren't as successful as normal queens – 86%of queens reared were dwarf queens, but they only headed 22% of the colonies that the team studied. Wenseleers and his co-workers suggest that this discrepancy may be due to workers actively discriminating against the dwarf queens.Many questions remain about this intriguing system in which individuals seem able to choose whether to become queens. In particular, it would be interesting to know exactly which mechanisms determine the fate of stingless bee larvae. What role does nutrition play in determining their fate? If nutrition determines whether an individual becomes a queen or a worker, then some larvae destined to become workers are apparently overcoming nutritional limits and becoming queens despite their limited food supply. What physiological mechanisms control nutritional responses in this system? Recent advances in Drosophila have given us some places to start looking for answers, such as insulin receptor pathways. Perhaps these or other, as yet unknown, pathways play a role in the production of dwarf queens in stingless bee colonies. Whatever the case, this system has the potential to keep ecologists and physiologists busy for many years to come.

A genetic component in the determination of worker polymorphism in the Florida harvester ant Pogonomyrmex badius

The interplay of genetic and environmental factors in the determination of social insect castes has long intrigued biologists. Though an overwhelming majority of studies establish that factors such as nutrition, pheromones and temperature determine the developmental fate of worker larvae, genetic components have recently been shown to play a role in the determination of morphological worker castes in leaf-cutting ants. Here we demonstrate that the determination of worker castes in the strongly polyandrous Florida harvester ant, Pogonomyrmex badius, has a genetic component. The overall distribution of caste members among patrilines in our study colonies is significantly different from the intracolonial caste ratio. Though this effect was not apparent in all colonies, our results suggest that workers of different patrilines in P. badius differ significantly in their propensities to develop into a certain worker caste. This genetic basis of worker polymorphism may go unnoticed in many social hymenopterans because of their low intracolonial genetic diversity due to monogamous colony structure. The worker polymorphism of P. badius is a taxonomic isolate and presumably a young trait in the genus. Therefore, a common genetic component of the determination of morphological and behavioral worker castes in social insects might be farranging taxonomically and may even be based on a genetic machinery inherent to all hymenopterans, but dormant in most.

Evolution of miniaturisation in inquiline parasitic ants: Timing of male elimination in Plagiolepis pygmaea, the host of Plagiolepis xene

Inquiline ant species are workerless social parasites whose queens rely completely on the host worker force to raise their brood. A remarkable feature of inquiline ants is the small size of sexuals, which are of the same size as or smaller than host workers. It has been suggested that miniaturisation evolved by parasites to prevent host workers from discriminating between their own worker brood and the inquiline sexuals, so that male and female inquilines can develop under conditions where the host species does not produce its own sexuals. In line with the miniaturisation hypothesis, workers of the ant P. pygmaea cull all the male brood of their own species, whereas at the same time males of their inquiline parasite P. xene are reared to adulthood. Here, we tested whether P. pygmaea workers recognize and eliminate males of their own species when they reach the size of the larger workers. Contrary to the assumption that size is indeed the primary cue used by workers to discriminate male from worker brood, we found that males of P. pygmaea are culled between the small and medium larval stages, that is much before reaching the critical size of the largest worker larvae. Based on this finding, we propose an extension of the miniaturisation hypothesis with a first step whereby the parasitic P. xene males escape the caste and sex recognition system of the host during early development. The most likely mechanism is chemical mimicry of host worker larvae. Miniaturisation would have evolved later to prevent the host workers to secondarily use size as a recognition cue to eliminate P. xene males.

A member of the short-chain dehydrogenase/reductase (SDR) superfamily is a target of the ecdysone response in honey bee (Apis mellifera) caste development

Many aspects in caste polyphenism result from hormonally controlled differential gene expression. A DDRT-PCR screen for ecdysteroid-regulated genes in ovaries revealed a set of ESTs coding for metabolic enzymes. For a cDNA encoding a short-chain dehydrogenase/reductase (SDR) we obtained the complete coding sequence (246 amino acids), revealing the protein motifs typical of insect SDRs. Its initially high expression in early fifth-instar larvae vanished in prepupae. Expression levels in worker larvae were higher than in queen larvae, suggesting negative regulation by the caste-specific ecdysteroid titer. This finding was confirmed by in vitro exposure of competent worker ovaries to makisterone A. In contrast to whole body RNA extracts, two SDR transcripts were detected in the ovaries. Both had their expression downregulated by makisterone A. Hormonal regulation and tissue-specific expression pattern makes this SDR an interesting enzyme for comparative molecular studies on social insect caste polyphenisms. Apis mellifera / differential display PCR / ecdysteroid / alcohol dehydrogenase / caste polyphenism

Determining the Content of Sulfohydryl Groups in Bee Brood Proteins

Unfortunately, bee brood – a nontraditional product of beekeeping – is still not used in domestic medicine because of insufficiently studied chemical composition and pharmacological activity. Comprising a mixture of bee queen, worker, and drone larvae, prepupas, and pupas, bee brood contains a complex of biologically active substances including amino acids, proteins, lipids, and mineral compounds [1 – 3]. Of special significance among these components are proteins offering a valuable source of both essential and nonessential amino acids. As is known, sulfohydryl groups enter into the composition of essential amino acids cysteine and cystine. It was established that the activity of bee brood enzymes involved in the synthesis of proteins and nucleic acids and in the synthesis and decomposition of ATP is also determined primarily by the presence of sulfohydryl groups in the active centers of enzyme molecules. In this context, we have developed a spectrophotometric technique and studied the content of sulfohydryl groups in the proteins of bee brood from several bee species. These species represented several types (gray Caucasus mountain, Italian, middle-Russian, and Carpathian) collected in Stavropol, Krasnodar, Rostov, and Irkutsk regions. We have also studied the reference samples of bee milk.

Differential tending of worker and drone larvae of the honey bee,Apis mellifera, during the 60 hours prior to cell capping

Varroa destructor is a parasitic mite of A. mellifera. Female mites reproduce on both drone and worker brood; but they are found 5-9 times more often on drone brood. We examined larval tending by brood nest bees to determine if this behavior could provide an explanation for these differences. We observed workers tending worker and drone larvae in three observation colonies commencing 60 h prior to the completion of cell capping, an interval that includes the susceptible periods of both types of brood. Workers invested 2.78 times more time tending drone larvae than worker larvae, a difference due primarily to a greater number of tending acts directed towards drone brood. This finding demonstrates that phoretic mites may have more opportunity to enter drone brood. The 2.78 fold difference in time tending larvae combined with the previously reported 2-4-fold differences in the lengths of the periods when cells are capped could explain a 5.56-11.12 fold difference in mite levels in worker and drone brood, similar to 'previously' reported differences. Varroa destructor / Apis mellifera / nurse bee / cell capping

Effect of queen quality on interactions between workers and dueling queens in honeybee ( Apis mellifera L.) colonies

The fitness of a social insect colony depends greatly on the quality (i.e., mating ability, fecundity, and offspring viability) of its queen(s). In honeybees, there is marked variation in the quality of young queens that compete in a series of lethal duels to replace a colony’s previous queen. Workers interact with queens during these duels and could increase their inclusive fitness by biasing the outcomes of the duels in favor of high-quality queens. We predicted that workers will have more antagonistic interactions (chasing, grabbing, clamping) and fewer beneficent interactions (feeding, grooming) with low-quality than high-quality queens. To test this prediction, we reared queens from 0-day-old, 2-day-old, and 3-day-old worker larvae in observation colonies undergoing queen replacement, thus producing high-quality, low-quality, and very low-quality queens, respectively. Immediately after each queen emerged, we observed her for 1 h to record her interactions with the workers. Subsequent morphological measurement of the queens confirmed that initial larval age had a significant effect on queen quality. However, there was no consistent effect of queen quality on the rates of worker–queen interactions, thus falsifying our hypothesis. The mean power of our tests was high (0.599), therefore the probability of a type II error (a false negative) is low. We conclude that if workers actively select high-quality queens, then they do so prior to queen duels, during queen development. We suggest that each worker–queen interaction has a distinct adaptive significance rather than forming a suite of behavior that favors particular queens (e.g., chasing repels any queen that approaches a queen cell, thus protecting all queen cells from destruction).

Differential feeding of worker larvae affects caste characters in the Cape honeybee, Apis mellifera capensis

Sections of brood from colonies of the Cape honeybee ( Apis mellifera capensis), the African honeybee ( A. m. scutellata), and hybrid bees of the two races were exchanged between colonies to study the effect of different brood-origin/nurse-bee combinations on development of caste characters. When Cape larvae were raised by African workers the amount of food provided almost doubled in comparison with Cape larvae reared by their own workers. In contrast, African larvae raised by Cape workers were provided with only half the amount they received from their own workers. After the bees emerged, we found a large degree of plasticity in characters related to caste differentiation, which corresponded closely to the amount of food provided. Super-fed Cape bees had enlarged spermathecae, were heavier than normal workers and developed more rapidly, and had reduced pollen combs, all typical for a more queen-like condition. Ovariole numbers did not appear to be enhanced by additional feeding. Cape bees that behave as social parasites in African bee colonies were most queen-like in the characters studied, albeit within the range that was found for Cape bees from normal colonies, suggesting within-colony selection for characters that enhance reproduction.

The organisation of larval feeding in bumblebees (Hymenoptera, Apidae) and its significance to caste differentiation

In pollen-storing bumblebees, the rate at which workers nourish larvae has been proposed to be the main factor influencing caste differentiation since workers feed prospective queens more frequently and longer than worker larvae during the last instars. In order to determine how the frequency of feedings is established small groups of Bombus terrestris larvae were either subjected to starvation or nourished regularly by workers. Experimentally starved larvae were fed significantly earlier and more often than control larvae. Behavioural observations provide evidence for the existence of a stimulus of larval origin that releases feeding behaviour in workers. Intentional inspections in the sense of a recognisable and functional behaviour intended to organise the feeding process were never observed. We argue that worker inspections are not required for the adequate maintenance of larvae and that a simple stimulus-response chain appears to be sufficient to regulate feeding behaviour at the individual and the colony level in bumblebees. Furthermore, hand-rearing experiments with female larvae indicate that queen rearing is not dependent on a high frequency of feedings in itself, nor is worker development induced by larval deprivation. This makes it unlikely that workers impose a caste-specific frequency of feedings on larvae in order to actively control or manipulate caste development. Since caste fate seems to be predetermined early in larval life, we propose that early caste-specific differences in development are reflected in the physiology of the larvae and the associated nutritional needs to which workers respond accordingly. Consequently, caste-specific differences in feeding frequencies are a result, but not the cause of differences in development.

Ovarian growth during larval development of queen and worker of Apis mellifera (Hymenoptera: Apidae): a morphometric and histological study.

The present work reports the differences between the ovarian grow in queen and worker larvae of A. mellifera, from the start of differential feeding. The observations made of the growth rates in larvae of both castes showed that the queen and worker larvae have the same rates of cephalic capsule growth from one instar to another but the weight gain is greater in queens. In the same way, the draw areas of ovaries of queens increase more and continuously, while from the 5th instar on the ovaries of workers decrease in size. The decrease is due to a loss of ovariole numbers that starts early in the worker larvae and increases in the 4th-5th instar. The ovarian shape in queens and workers became different in the last larval instars.

Effects of dietary transgenic Bt corn pollen on larvae of Apis mellifera and Galleria mellonella

SUMMARYThe effects of dietary transgenic Bt corn pollen on 4–5–day-old honey bee worker larvae were examined. We measured larval and pupal mortalities, pupal weight, and haemolymph protein concentration of newly emerged adults after they were fed (as larvae) various pollens (mixed bee pollen, non-transgenic corn pollen, Cry1A(b) or Cry1F corn pollen). There were no significant differences in all the parameters tested between larvae fed transgenic Bt corn pollen Cry1 A(b) or Cry1 F and non-transgenic corn pollen. We also evaluated Bt corn pollen as a potential control for wax moth larvae in a laboratory study. We fed first instar wax moth larvae three types of pollen: non-transgenic corn pollen, Cry1 A(b) corn pollen, and Cry1F corn pollen. We found that the mortality of larvae fed Cry1 F corn pollen was significantly greater than the mortality of larvae fed Cry1 A(b) corn pollen or non-transgenic corn pollen (P < 0.05). In each trial Cry1 F fed larvae showed 100% mortality. Our studies suggest that transgenic Bt corn pollen does not pose a threat to honey bee larval development and has the potential to serve as an alternative control for wax moth.

Thermal characterization of social vespid silk

In this paper we try to establish a link between the microclimate in the wasp nest and the structure and thermal stability of vespid silk. We suggest that there are at least two types of water that is absorbed by the silk of Oriental hornets, namely, surface water and intrinsic structural water. The release of both types of water was found to be reversible. The enthalpy values of the endothermic peaks associated with the release of water from different silk samples do not differ substantially and are in the range of 106 to 130 J g-1 for the Vespa orientalis male larvae silk (sample #1), Paravespula germanica (yellowjacket) worker larvae silk (#3) and Vespa orientalis nest envelope(#4). For the Vespa orientalis worker larvae silk (sample #2), however, it is twice as large (228 J g-1). This is in agreement with the increased total amount of absorbed water. The silk studied has a fibrilar structure with interconnecting surfaces overlying entire regions. It is assumed that the initial water loss stems from water evaporation from the coat of the fibers - a daily occurrence in the hornets' nest. Heating to above 70°C may result in structural changes in the silk core.

Cuticular volatiles, attractivity of worker larvae and invasion of brood cells by Varroa mites. A comparison of Africanized and European honey bees

Africanized honey bees (AHBs) of Brazil and Mexico have proven to be tolerant to Varroa destructor mites. In contrast, European honey bees (EHBs: Apis mellifera carnica) at the same tropical study site are highly intolerant to these ectoparasites. A lower attractiveness of Varroa-tolerant AHB larvae has been hypothesised to be an important trait in reducing the susceptibitlity of AHBs to these mites. Thus, selection for EHB brood that is less attractive to mites is thought to be one possibility for limiting mite population growth and thus increase the tolerance of EHBs to the mite.¶In Ribeirao Preto, Brazil, European A. m. carnica bees and AHBs were tested with respect to their rate of brood infestation and brood attractiveness to Varroa mites. For the comparison of brood infestation rates, we introduced combs with pieces of EHB and AHB brood into honey bee colonies (18 repetitions). The relative infestation rate of EHB brood was significantly higher compared to AHB brood.¶The preference behaviour of single Varroa mites was tested in a laboratory bioassay where either living host stages were offered or host extracts were presented on dummies. By these tests we could confirm the preference of Varroa females for certain developmental host stages and for their corresponding extracts. In contrast to the within-colony results, Varroa mites in the laboratory bioassay showed a slight preference for AHB compared to EHB larvae.¶The gas chromatographic analysis revealed differences in the chemical spectrum of extracts obtained from different larvae. In accord with the results of the bioassays, we could detect stage-specific odour differences in larval cuticular compounds, including methyl esters and hydrocarbons that have been described as kairomones. None of these substances, however, revealed significant race-specific differences. Therefore, the quantity and composition of certain cuticular compounds seem to be responsible only for the recognition of a suitable host stage by Varroa females. The different infestation rates in the colonies, however, seem to be caused neither by race-specific differences in attractiveness of bee larvae nor by an extended attractive period of EHB larvae: both AHB and EHB larvae become attractive approximately 21 h before capping of the brood cell, and thus have the same window of time when they can be parasitised.¶Therefore differential Varroa-infestation rates are not related to larval attraction but probably are determined by other race-specific and colony-related factors.

Juvenile-hormone-dependent interaction of actin and spectrin is crucial for polymorphic differentiation of the larval honey bee ovary.

Programmed cell death in the worker ovary of Apis mellifera reduces the number of ovarioles during metamorphosis from 150-200 primordia to less than 10. In contrast, practically all ovarioles in the ovary of queens survive to the adult stage. The correct formation and persistence of polyfusomes has been suggested as a critical factor for ovariole survival. We have analyzed the developmental dynamics of F-actin and alpha-spectrin in fusomes of queen and worker larvae, and in juvenile-hormone-treated worker larvae. Small fusomes containing actin and spectrin can be detected in the ovaries of fourth instar larvae in both castes. After molting to the fifth instar, the actin-spectrin association persists in the enlarged fusomes of queen ovarioles. In workers, actin dissociates from the fusomal and cortical alpha-spectrin. Coinciding with the appearance of apoptosis markers, large agglomerates of actin are detectable in worker ovarioles. Treatment of fourth-instar worker larvae with juvenile hormone rescues ovarioles from apoptosis and maintains the actin-spectrin association. Juvenile-hormone-dependent actin-spectrin interaction is thus one of the earliest steps in the differentiation of a polymorphic ovary. Plasticity in ovariole numbers as a result of hormone-dependent fusome formation may be a more widespread phenomenon in insects, extending beyond caste polymorphism in highly eusocial Hymenoptera.

Effects of western honey bee (Hymenoptera: Apidae) colony, cell type, and larval sex on host acquisition by female Varroa destructor (Acari: Varroidae).

Female mites of the genus Varroa reproduce on the immature stages of Apis cerana F. and A. mellifera L. Mites are found more often in drone brood than worker brood, and while evolutionary explanations for this bias are well supported, the proximate mechanisms are not known. In one experiment, we verified that the proportion of hosts with one or more mites (MPV, mite prevalence value) was significantly greater for drones (0.763 +/- 0.043) (lsmean +/- SE) than for workers (0.253 +/- 0.043) in populations of mites and bees in the United States. Similar results were found for the average number of mites per host. In a second experiment, using a cross-fostering technique in which worker and drone larvae were reared in both worker and drone cells, we found that cell type, larval sex, colony and all interactions affected the level of mites on a host. Mite prevalence values were greatest in drone larvae reared in drone cells (0.907 +/- 0.025), followed by drone larvae reared in worker cells (0.751 +/- 0.025), worker larvae reared in worker cells (0.499 +/- 0.025), and worker larvae reared in drone cells (0.383 +/- 0.025). Similar results were found for the average number of mites per host. Our data show that mite levels are affected by environmental factors (cell type), by factors intrinsic to the host (sex), and by interactions between these factors. In addition, colony-to-colony variation is important to the expression of intrinsic and environmental factors.

Size Dimorphism in Bumblebees: a Result of Caste Specific Differences in Fat Body Metabolism?

Most highly social insects exhibit a distinct size dimorphism between the female castes: queens being larger than workers. Nutritional factors are known to influence caste specific development to a great extent, but little is known about this in bumblebees. To test whether size differentiation in Bombus terrestris results from differences in food intake and fat body metabolism, as was suggested in earlier research, we compared the weight gain of the midgut and the fat body, as well as its general composition for last instar queen and worker larvae. The present results show that the relative growth rates and the general composition of the fat body in queen and worker larvae are similar, indicating that caste specific size dimorphism is not owing to differences in fat body metabolism. The timing of the onset of defecation appeared to have a pronounced effect on the determination of midgut weight and body mass, and it is discussed that this might have complicated the correct interpretation of earlier results.