Timing Of Larval Development Research Articles

Large larvae of a flush-feeding moth (Epirrita autumnata, Lepidoptera: Geometridae) are not at a higher risk of parasitism: implications for the moth's life-history

The effect of larval body size of Epirrita autumnata (Lepidoptera, Geometridae) on the risk of parasitism was studied in a field experiment. The experiment involved three pairwise exposures of different larval instars to parasitoids. Three hymenopteran species were responsible for most of the parasitism. Parasitism risk was found to be host-instar independent. This result was consistent across parasitoid species and experiments. The results suggest that host use by larval parasitoids cannot constrain selection for larger body size in E. autumnata. However, high mortality due to parasitism may select for a short developmental period (the slow-growth/high-mortality hypothesis), and smaller body sizes as a by-product. A strong selective effect of parasitism on the timing of larval development in E. autumnata is also unlikely. The larger was the host, the larger was the adult size of the parasitoid and the shorter its development time (for one species). We suggest that the lack of a preference-performance linkage in the system studied may be related to the time stress associated with the short phenological window of host vulnerability.

Food availability controls the onset of metamorphosis in the dung beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

SummaryIn nature, larvae of the dung beetle Onthophagus taurus (Schreber 1759) are confronted with significant variation in the availability of food without the option of locating new resources. Here we explore how variation in feeding conditions during the final larval instar affects larval growth and the timing of pupation. We found that larvae respond to food deprivation with a reduction in the length of the instar and premature pupation, leading to the early eclosion of a small adult. To achieve pupation, larvae required access to food for at least the first 5 days of the final instar (= 30% of mean third‐instar duration in control individuals), and had to exceed a weight of 0.08 g (= 58% of mean peak weight in control individuals). Larvae that were allowed to feed longer exhibited higher pupation success, but increased larval weight at the time of food deprivation did not result in increased pupation success except for larvae weighing > 0.14 g. Larvae responded to food deprivation by initiating and undergoing the same sequence of developmental events, requiring the same amount of time, as ad libitum‐fed larvae once those had reached their natural peak weight. Our results reveal a striking degree of flexibility in the dynamics and timing of larval development in O. taurus. They also suggest that premature exhaustion of a larva's food supply can serve as a cue for the initiation of metamorphosis. Premature metamorphosis in response to food deprivation has been documented in amphibians, but this is, to the best of our knowledge, the first time such a behaviour has been documented for a holometabolous insect. We discuss our findings in the context of the natural history and behavioural ecology of onthophagine beetles.

Effects of 4-n-nonylphenol and 17beta-oestradiol on early development of the barnacle Elminius modestus.

Pollutants that are present in the aquatic environment and cause abnormal endocrine function in wildlife populations have been termed endocrine disrupting chemicals (EDCs). The impacts of these chemicals on the reproduction and development of vertebrates has been shown to be significant in both field studies and laboratory experiments. Over the past decade the number of investigations into the impacts of EDCs that affect reproductive and sexual characteristics (reproductive EDCs) has increased and evidence of their potency is evident in numerous wildlife species and through data from in vitro tests. However, little information is available on whether chemicals which act as EDCs in vertebrate species affect aquatic invertebrates. The case of imposex in archeogastropods following exposure to tributyltin (TBT) is a notable exception. Moreover, a number of studies have shown that development, fecundity and reproductive output of some aquatic invertebrates are affected significantly by exposure to pollutants. In order to determine whether external signs of exposure to vertebrate EDCs can be observed and monitored in invertebrate species, we exposed larvae of the barnacle Elminius modestus to environmentally realistic concentrations of the xeno-oestrogen, 4-n-nonylphenol (NP), and the natural oestrogen, 17beta-oestradiol (E(2)). Early life stages (nauplii and cyprids) were also exposed in the laboratory to determine whether there were effects on the timing of larval development and settlement. Ovary development and size of juveniles was measured following chronic exposure. Exposure to NP in the concentration range 0.01-10 µg l(-1) resulted in disruption of the timing of larval development. Similar results were obtained with E(2). Pulse exposures showed that the timing of exposure is critical and exposures for a period of 12 months caused long-term effects. A linear, concentration-dependent response was not evident.

Histological study of the development of the embryo and early larva of Oreochromis niloticus (Pisces: Cichlidae).

The developmental stages of Oreochromis niloticus are similar to those described in other mouth-breeding tilapias except that, as in zebrafish, no cavity was found in the blastula. Variation in the rate of development of the embryo and larva of O. niloticus was found within a clutch of eggs as well as between clutches. Hatching glands are described for the first time in tilapias. They are widely distributed within the ectoderm covering the head, body, tail, and surface of the yolk sac near its attachment to the embryo. Timing of larval development is similar to that in other mouthbrooding tilapias, but is slower than that found in substrate-spawning tilapias. A pneumatic duct connects the swimbladder to the digestive tract and swimbladder inflation and initiation of feeding occurs at about the same time. The digestive tract of the larva 8 and 9 days after fertilization is similar to that found in the adult, except that there are no digestive glands. An endocrine pancreatic islet was first seen 76 h after fertilization. A prominent thymus gland is present at 100 h. Hematopoietic tissue develops in the vicinity of the pronephros during early larval development. A spleen develops later, 7 days after fertilization.

Effects of temperature on energy cost and timing of embryonic and larval development of the terrestrially breeding moss frog, Bryobatrachus nimbus.

The Australian moss frog, Bryobatrachus nimbus, oviposits four to 16 large eggs in terrestrial nests constructed in moss or lichen in subalpine regions of southern Tasmania. Nidicolous larvae overwinter beneath snow, reaching metamorphosis without feeding after 395 d, the longest development time known for an endotrophic anuran. However, a few clutches develop more quickly and metamorphose before winter. This study examines the effect of temperature on development time and energy expenditure by measuring temperatures and developmental stages in field nests as well as rates of oxygen consumption (Vo2), developmental stage, body mass, and energy content in the laboratory at three relevant temperatures (5 degrees, 10 degrees, 15 degrees C). Eggs and larvae reared at 5 degrees C differentiated very slowly, and their development time far exceeded those in natural nests, but development times at 10 degrees and 15 degrees C averaged 277 and 149 d, respectively, and were shorter than field incubation times. Generally, respiration rates of aquatic hatchlings were low in comparison with other species but increased with larval age and jumped about 25% higher near metamorphosis when larvae were able to air breathe. The mean energy density was 26.0 J mg(-1) for the dry ova and 20.6 J mg(-1) for a dry gut-free froglet, and total production efficiency was 61.5%. We developed a model based on the relationships between incubation temperature and V&d2;o2 to estimate the respiratory cost of development to metamorphosis, the first such study for an amphibian. The cost was 177 J at 15 degrees C, 199 J at 10 degrees C, and at least 249 J at 5 degrees C, and we predicted that continual development at 5 degrees C would lead to premature yolk depletion because it equalled the 249 J contained in fresh ova. Continuously logged field-nest temperatures and interpolation of laboratory data provided estimates of development rates, Vo2, and respiratory energy costs in field nests. Development to metamorphosis required between 185 and 234 J when larvae overwintered, but completion of metamorphosis before winter saved 123 J. However, the advantage of emergence in warmer months, when conditions are suitable for feeding and growth, may offset the greater energy cost of overwintering.

Temporal and spatial variability in settlement and recruitment of echinoderms in kelp beds and barrens in Nova Scotia

Settlement and recruitment of echinoderms were measured in 2 adjacent habitats: a kelp bed (Laminaria longicruris) and an echinoid (Strongylocentrotus droebachiensis)-dominated barren ground at each of 2 sites in the rocky subtidal zone (5 to 10 m depth) off Nova Scotia. Settle- ment was measured using artificial collectors made of 0.05 m 2 panels of plastic turf that were deployed 0.2 m (low) and 2.3 m (high) above the bottom in each habitat, and sampled bi-weekly from June to November 1992 to 1994. Distinct settlement pulses of ophiuroids (Ophiopholis aculeata and Ophiura spp.), asteroids (Asterias spp.) and an echinoid (S. droebachiensis) occurred between July and September of each year at both sites. Timing of settlement differed consistently among species in relation to differences in timing of spawning and larval development. The magnitude of each pulse varied between years and species, and the year of maximum settlement differed between species, suggesting that species-specific processes regulate settlement rather than general environmental conditions. Settlement of all species was greater at the more protected site, but between-habitat pat- terns were not consistent among species. Asteroids settled in greater numbers in and above kelp beds, whereas ophiuroids showed a trend toward greater settlement in the barrens. The echinoid S. droebachiensis also tended to settle more in barrens, although the difference was not statistically sig- nificant. This suggests that kelp beds do not exhibit a consistent or strong effect on settlement. Sam- pling settlement concurrently at different frequencies (3 to 48 d) gave different estimates of settle- ment, indicating a need for assessment of sampling artifacts (e.g. changes in collector quality, post-settlement mortality or migration) that can occur over longer deployment intervals. For most species sampled, settlement predicted recruit density in natural populations the following year. How- ever, the strength of the relationship varied between species, probably because of differing post- settlement processes.

Effects of Autumn Temperature and Oviposition Date on Timing of Larval Development and Risk of Parasitism in a Spring Folivore

Epirrita autumnata (Lepidoptera, Geometridae) is a periodically fluctuating spring folivore which overwinters in the egg stage. The timing of its larval development can be highly variable, and the synchrony of larvae with parasitoids may affect their risk to be parasitized. We estimated the effects of both oviposition date and site on the timing of larval development by allowing experimental eggs, which were laid on various dates during the oviposition period, to overwinter both above and below snow level, and on both the northern and southern sides of tree trunks. Accumulation of thermal sum was used to quantify the effect of oviposition date on the timing of egg hatch. To assess whether the timing of larval development affected the risk of parasitism, the larval stages susceptible to parasitism and the length of the parasitoid flight period were detennined by exposing host larvae to parasitoids. Unlike previously assumed, the thermal sum required for egg hatching began to accumulate already in autumn, and autumn and spring temperatures additively determined the timing of egg hatch. The first-laid eggs accumulated 70% of the thermal sum required for hatching in autumn whereas the last-laid eggs accumulated less than 3%. Accordingly, because the oviposition period of E. autumnata is long and its timing is variable between years, the period of egg hatching also is long and can be variable between years. However, the flight periods of most parasitoid species exceeded the spread of hatching times and developmental stages of larvae. Only during years of low parasitoid density may the variability in egg hatching times create a temporal refuge for some larvae, contributing to the increase of E. autumnata populations.