Anterior Body Segments Research Articles

Does defensive posture increase mimetic fidelity of caterpillars with eyespots to their putative snake models?

Abstract Organisms often evolve behaviours that increase or reinforce the protection from predators afforded by their morphological defences. For example, mimetic animals may adopt postures or locomotory behaviours that emulate a characteristic feature of their model to increase predator deception. Caterpillars with eyespots are thought to mimic snakes, and when threatened many of these caterpillars adopt a posture that appears to enhance this resemblance. Herein we evaluate the quantitative strength of evidence of behavioural mimicry in the caterpillars of 14 species by comparing how closely a series of putative snake-mimicking caterpillars resemble snakes while at rest and when threatened. Specifically, we quantified the head morphology and eye position of a range of snake species, as well as the shape of the apparent head (i.e. anterior body segments) and position of eyespots in caterpillars resting or in their defensive posture. This allowed us to objectively examine evidence for an increased resemblance to either snakes generally, or to Viperidae snakes specifically, upon adopting the defensive posture. Widening the anterior body segments during the defensive posture typically made caterpillars appear more viper-like as opposed to more snake-like in general. Enhanced resemblance to vipers upon mounting the defensive posture was apparent only from the dorsal view. Laterally, caterpillars more closely resembled snakes in the resting posture and shifting to the defensive posture instead reduced mimetic fidelity. Overall we found evidence for behavioural mimicry in all 14 species examined. We highlight that objectively quantifying mimetic fidelity can help identify key features involved in deception.

The position of eyespots and thickened segments influence their protective value to caterpillars

It has long been believed that the paired circular markings (“eyespots”) of Lepidoptera larvae, combined with their defensive postures (where the larvae swell their anterior body segments) protect them from potential predators. These traits could inhibit attacks by enhanc-ing the similarity of the prey item to the predator’s own natural enemies (notably snakes), but alternatively, they may simply exploit the predator’s wariness of novel and/or conspicuous objects. To differentiate between these contrasting explanations, we evaluated the responses of naive chicks (

Defensive posture and eyespots deter avian predators from attacking caterpillar models

Many lepidopteran caterpillars appear to gain protection from insect-eating birds because they possess eyespots, a pair of conspicuous markings on the body generally thought to resemble the eyes of a predator. Similarly, many caterpillars widen their anterior body segments when threatened, and this defensive posture may also deter attack because it emphasizes the caterpillar's eyes and/or allows the caterpillar to resemble a snake more closely. Yet, given that this behaviour is not limited to caterpillars with eyespots, it is possible that the defensive posture may serve as a predator deterrent in itself. To disentangle the protective value of eyespots and widened anterior body segments, we created four types of artificial caterpillar in a 2×2 factorial design and deployed them on tree branches in the field where they were exposed to predation attempts by wild birds. The ‘survivorship’ of the artificial caterpillars was monitored over 90 h in six locations. We found a clear survival advantage for artificial prey with eyespots or a widened head shape. However, prey possessing both eyespots and a widened head did not survive longer than prey with either characteristic alone, suggesting that the traits did not act synergistically to deter attack. Interestingly though, only caterpillars in the eyespot-defensive posture treatment received more pecks directed towards the anterior ‘head’ region. Collectively, this study demonstrates clearly that caterpillars possessing either eyespots or widened anterior body segments are likely to experience reduced predation by wild birds, even at northern latitudes where there are relatively few arboreal snakes.

Coming apart at the seams: morphological evidence for pregnathal head capsule borders in adult Tribolium castaneum

Cephalization and seamless fusion of the anterior body segments during development obscure the segmental boundaries of the insect head. Most of the visible seams are thought to reflect cuticular infolding for structural reinforcement rather than a merger of cuticular plate borders. Incomplete fusions and other modifications of the adult head found in eight Tribolium mutations indicate that the frontal and gular sutures likely are true sutures that mark borders between adjacent cuticular plates, and suggest that the anterior facial shelf is a composite of three independent cuticular surfaces: ocular, antennal, and clypeo-labral. Additionally, midline splits of the clypeo-labrum and gula, and membranous lesions on the lateral head capsule reveal probable borders of adjacent cuticular plates where visible sutures are normally absent. The anterior lateral lesions seen in the Lucifer mutation mark a border between ocular and antennal plates and appear to identify part of the postfrontal sutures. While revealing or clarifying possible intersegmental borders between ocular, antennal, and clypeo-labral plates, the various modified or unfused surfaces of the head neither reveal an additional acronal plate nor support the view that the clypeo-labrum is segmentally associated with ocular cuticle.

Two Alternating Motor Programs Drive Navigation in Drosophila Larva

When placed on a temperature gradient, a Drosophila larva navigates away from excessive cold or heat by regulating the size, frequency, and direction of reorientation maneuvers between successive periods of forward movement. Forward movement is driven by peristalsis waves that travel from tail to head. During each reorientation maneuver, the larva pauses and sweeps its head from side to side until it picks a new direction for forward movement. Here, we characterized the motor programs that underlie the initiation, execution, and completion of reorientation maneuvers by measuring body segment dynamics of freely moving larvae with fluorescent muscle fibers as they were exposed to temporal changes in temperature. We find that reorientation maneuvers are characterized by highly stereotyped spatiotemporal patterns of segment dynamics. Reorientation maneuvers are initiated with head sweeping movement driven by asymmetric contraction of a portion of anterior body segments. The larva attains a new direction for forward movement after head sweeping movement by using peristalsis waves that gradually push posterior body segments out of alignment with the tail (i.e., the previous direction of forward movement) into alignment with the head. Thus, reorientation maneuvers during thermotaxis are carried out by two alternating motor programs: (1) peristalsis for driving forward movement and (2) asymmetric contraction of anterior body segments for driving head sweeping movement.

Whistling in caterpillars (Amorpha juglandis, Bombycoidea): sound-producing mechanism and function

Caterpillar defenses have been researched extensively, and, although most studies focus on visually communicated signals, little is known about the role that sounds play in defense. We report on whistling, a novel form of sound production for caterpillars and rare for insects in general. The North American walnut sphinx (Amorpha juglandis) produces whistle 'trains' ranging from 44 to 2060 ms in duration and comprising one to eight whistles. Sounds were categorized into three types: broadband, pure whistles and multi-harmonic plus broadband, with mean dominant frequencies at 15 kHz, 9 kHz and 22 kHz, respectively. The mechanism of sound production was determined by selectively obstructing abdominal spiracles, monitoring air flow at different spiracles using a laser vibrometer and recording body movements associated with sound production using high-speed video. Contractions of the anterior body segments always accompanied sound production, forcing air through a pair of enlarged spiracles on the eighth abdominal segment. We tested the hypothesis that sounds function in defense using simulated attacks with blunt forceps and natural attacks with an avian predator - the yellow warbler (Dendroica petechia). In simulated attacks, 94% of caterpillars responded with whistle trains that were frequently accompanied by directed thrashing but no obvious chemical defense. In predator trials, all birds readily attacked the caterpillar, eliciting whistle trains each time. Birds responded to whistling by hesitating, jumping back or diving away from the sound source. We conclude that caterpillar whistles are defensive and propose that they function specifically as acoustic 'eye spots' to startle predators.

Maternal Torso Signaling Controls Body Axis Elongation in a Short Germ Insect

In the long germ insect Drosophila, all body segments are determined almost simultaneously at the blastoderm stage under the control of the anterior, the posterior, and the terminal genetic system . Most other arthropods (and similarly also vertebrates) develop more slowly as short germ embryos, where only the anterior body segments are specified early in embryogenesis. The body axis extends later by the sequential addition of new segments from the growth zone or the tail bud . The mechanisms that initiate or maintain the elongation of the body axis (axial growth) are poorly understood . We functionally analyzed the terminal system in the short germ insect Tribolium. Unexpectedly, Torso signaling is required for setting up or maintaining a functional growth zone and at the anterior for the extraembryonic serosa. Thus, as in Drosophila, fates at both poles of the blastoderm embryo depend on terminal genes, but different tissues are patterned in Tribolium. Short germ development as seen in Tribolium likely represents the ancestral mode of how the primary body axis is set up during embryogenesis. We therefore conclude that the ancient function of the terminal system mainly was to define a growth zone and that in phylogenetically derived insects like Drosophila, Torso signaling became restricted to the determination of terminal body structures.

Body elongation and decreased reproductive output within a restricted clade of lizards (Reptilia: Scincidae).

Relationships between body shape and relative abdominal size were compared among differentially elongate species within the scincid lizard genus Brachymeles, to investigate how morphological evolution affects the proportion of body volume available to hold eggs and offspring. Relative abdominal size is inversely related to elongation, suggesting that relative clutch mass decreases with addition of abdominal body segments. Shape‐volume relationships contradict trends seen in comparisons among distantly related limbed and limbless squamates (lizards and snakes), in which snakes have relatively more abdominal volume. Comparison within a phylogenetically restricted group allows the identification of functional and ontogenetic factors potentially limiting reproductive output. In Bruchymeles, constraining factors include retention of anterior body segments bearing parasternal ribs, which prevents extension of the clutch anteriorly within the body, and reduction of allometry of abdominal segments, which provides extended series of uniformly‐sized vertebrae for limbless locomotion, but reduces the relative size of the abdomen. The latter trait is associated with overall size reduction, which affects relative egg‐size and packing. Factors constraining abdominal volume in this genus are probably common to other elongate lizards, a morphological group that has been rarely represented in comparative studies of life history.

Developmental Anomalies in Habrobracon hebetor Exposed to Volatilized Agents

Insect anesthetics caused developmental damage during the third instar of larvae of the wasp, Habrobracon hebetor Say. Results obtained with a recently introduced agent, triethylamine (TEA), were compared with the effects of the conventional anesthetics ether and chloroform. All three compounds disturbed cocoon-spinning behavior and decreased silk production. Also, developmental delay was common. A linear relationship between the duration of exposure and survival to adulthood was demonstrated. The main effect on morphology from TEA was upon antennal length and segmentation. After exposure to ether and chloroform, anterior body segments and appendages failed to develop. In addition, microcephaly, microophthalmia, anophthalmia, and missing mouthparts were induced.

Homoeosis in Drosophila: A description of the polycomb lethal syndrome

Adults heterozygous for dominant mutations at the haploabnormal Polycomb ( Pc) locus display many homoeotic transformations. E. B. Lewis (1978) first described the cuticular morphology of lethal embryos homozygous for Polycomb mutant alleles, and suggested that the Pc + gene product acts as a repressor of genes in the Bithorax gene complex. In the present work, we have further examined the Polycomb lethal syndrome by phase-contrast and scanning electron microscopy of whole mounts, and show that the phenotype is more complex than hitherto realized. Many cuticular features characteristic of more anterior body segments are partially or completely transformed to resemble those of more caudal segments; the posteriormost homoeotic features which develop are those of the 8th abdominal segment (rather than the 9th or 10th). Involution and dorsal closure of the head are usually incomplete, and the labium and dorsal head regularly develop cuticular features normally characteristic of abdominal segments. Not all homoeotic alterations are caudal, however, and embryos also display partial transformations of the 9th (or 10th) abdominal segment to 8th and of meso- and metathorax to prothorax. It should be emphasized that Polycomb mutations do not homoeotically transform segments as a whole; various cuticular markers differ considerably in their relative probability and extent of transformation, and a single segment may display features normally specific to several different segments. We suggest that these mutations result in an instability in the transmission of determined states.

The segmental origin of the appendages of the head and anterior body segments of a spiroboloid milliped, Narceus annularis.

AbstractStudy of a series of embryos showed that the spiroboloid leg arrangement (1 pair of legs on each of the first 5 segments) is derived from the typical leg arrangement (no legs on segment 1, 1 pair on segments 2 through 4, and 2 pairs on segment 5) by a shifting forward one segment of the first four pairs of legs.A careful re‐examination of the literature, especially papers by Robinson ('07), Silvestri ('03, '49), Pflugfelder ('32), and Manton ('61), combined with observations of Narceus embryos led to the conclusion that (1) the anterior body segments are primatively single (2) the gnathochilarium is composed of only one pair of mouthparts, the diplopod head having but two gnathal segments, and (3) the intercalary segment is present in the Diplopoda.

Classical conditioning of the extension response in the earthworm☆

Abstract Earthworms were observed to extend the anterior body segments reliably during the presentation of a strong vibratory stimulus. The response was classically conditioned and subsequently extinguished. Backward conditioning and contiguity control groups indicated that learning was possible only with a forward conditioning order of stimuli. The extension response was observed to be relatively unaffected by central nervous system lesions leading to the conclusion that future studies of the role of the earthworm nervous system in behavior utilize this response rather than the contraction response to bright light which is reversed by lesions.

The Gross Morphology of the Anterior Nervous System of Nephtys

ABSTRACT The supra-oesophageal ganglion of Nephtys varies considerably in its size and gross morphology from one species to another. The ganglion is relatively larger in small species and then extends into the anterior body segments. It is epidermal and in contact with the cuticle in the prostomium, sub-epidermal in the anterior segments. In a number of species a pair of conspicuous posterior lobes to the ganglion has been developed. There is a tendency, particularly in the smaller worms, for the ganglion to be bifurcated at its posterior, and sometimes also at its anterior margin. The sensory nerves are as follows : tegumentary nerves, extremely variable in number and disposition, arising from the front of the ganglion; two pairs of antennary nerves arising immediately above the roots of the circum-oesophageal connectives, but generally distinct from them; and a pair of anterior optic ‘nerves’ supplying the paired, single-celled photoreceptors. The only other nerves arising from the ganglion are the nuchal nerves which arise from it opposite the nuchal organs, whatever the position of the brain. Since the latter may barely protrude into the prostomium, these nerves emerge from the anterior part of the brain in some species, instead of near the posterior end of the ganglion. The circum-oesophageal connectives are extremely long because the sub-oesophageal ganglion lies in the fifth segment. In their passage through the lateral lips of the mouth and along the sides of the guiar membrane, they give off three pairs of stomatogastric nerves and the segmental nerves of the first two segments. The third and fourth segmental nerves arise from the front of the sub-oesophageal ganglion, those of the fifth segment from the back of the ganglion. The stomatogastric nervous system is also described. The entire nervous system shows little specialization compared with that of most other errant polychaetes.

Report on a Collection of Branchiobdellids

Cambarincola macrocephela n. sp. (Figs. 1 and 2) Description: Cambarincola: Worm cylindrical, head large, expanded so that it is distinctly wider than anterior body segments. Head separated by deep constriction from Segment I. Dorsal lip with four finger-like projections, ventral lip entire. Body segments increasing in width from I to VII, posteriorly decreasing to the terminal sucker. Major annulations of body segments not elevated over minor annulations. Ventral and dorsal jaws similar, appearing as large triangular blocks of chitin terminating in sharp tooth. Without lateral teeth but margin uneven. Width of jaws at base 400 ,R. Pharyngeal diverticula four, two dorsal and two ventral. Alimentary canal in medial portion straight, with large sacculation in segments II and III and small sacculations in segments IV, V, VI and VII. Posterior to segment VII, narrows rapidly to anus in dorsal portion of segment X. Testes in pairs in segments V and VI. Vasa deferentia joining atrium in VI; with accessory sperm tube. Ovaries in segment VII. Length of mature worm 2.0 to 4.0 mm. Locality: Polecat Creek, Northern Teton County, Wyoming, August 16, 1941. Coll. Robert C. Brown. Host: Astacus gambellii. Holotype specimen: U. S. Nat. Mus. Coll. No. 20598.