Insect walking techniques on thin stems

The attachment ability of insects on surfaces are associated not only with the micro- and nanostructure of the adhering part of an attachment device, but also with the global scale kinematics responsible for contact formation and release. In the present study, the locomotory techniques of several representatives of insects from four different orders (Orthoptera, Heteroptera, Coleoptera, and Hymenoptera), possessing different types of attachment structures, are described. The study is based on video recordings of insects walking on a flat surface and on cylindrical rods of various thickness, imitating plant stems. Attachment devices of tarsi and pretarsi were visualized using Scanning Electron Microscopy. The results show a different manner in the use of adhesive structures on substrates with various curvatures. Insects bearing attachment pads on proximal tarsomeres usually touch flat and curved substrates using all tarsomeres, whereas insects with their attachment devices on the distal tarsomeres usually walk on flat surfaces using the distal tarsomeres of the overextended tarsus. On substrates, with diameters comparable to or larger than the tarsus length, insects walk above the stem by clasping the stem with the bent tarsi. On thin stems, insects clasp the stem between their tarsi and hang under the stem. Thus, on thin and thick rods, forces applied to attachment organs act in opposite directions. There are two methods of leg positioning for walking on a rough flat substrate. In the first case, the tarsus is straightened and the rough substrate is gripped between the claws and the proximal complex of attachment devices (tarsal euplantulae, fossulae spongiosa, and terminal spurs of tibiae). In the second case the tibia does not touch the substrate; the insect is supported only by distal tarsomeres. The tarsus is in an overextended condition. On rods, with diameters comparable to or larger than the tarsus length, insects walk by clasping the stem with the bent tarsi. This posture is characteristic for the majority of insects independent of the tarsal position they normally use while walking on a plane. If the rod’s diameter is smaller than the tarsus length, walking insects usually clutch it between contralateral tarsi. Using such a posture they are supported by interlocking or by strong friction, generated by attachment devices of the proximal tarsomeres, and do not use attachment devices of the pretarsus. Contact with the substrate is reinforced due to the coordinated contralateral clutch using all supporting legs. It is concluded that the use of different types of attachment structures correlates with locomotory techniques. Handling Editor: Heikki Hokkanen     

Evolution of nubbin function in hemimetabolous and holometabolous insect appendages

Insects display a whole spectrum of morphological diversity, which is especially noticeable in the organization of their appendages. A recent study in a hemipteran, Oncopeltus fasciatus (milkweed bug), showed that nubbin (nub) affects antenna morphogenesis, labial patterning, the length of the femoral segment in legs, and the formation of a limbless abdomen. To further determine the role of this gene in the evolution of insect morphology, we analyzed its functions in two additional hemimetabolous species, Acheta domesticus (house cricket) and Periplaneta americana (cockroach), and re-examined its role in Drosophila melanogaster (fruit fly). While both Acheta and Periplaneta nub-RNAi first nymphs develop crooked antennae, no visible changes are observed in the morphologies of their mouthparts and abdomen. Instead, the main effect is seen in legs. The joint between the tibia and first tarsomere (Ta-1) is lost in Acheta, which in turn, causes a fusion of these two segments and creates a chimeric nub-RNAi tibia–tarsus that retains a tibial identity in its proximal half and acquires a Ta-1 identity in its distal half. Similarly, our re-analysis of nub function in Drosophila reveals that legs lack all true joints and the fly tibia also exhibits a fused tibia and tarsus. Finally, we observe a similar phenotype in Periplaneta except that it encompasses different joints (coxa–trochanter and femur–tibia), and in this species we also show that nub expression in the legs is regulated by Notch signaling, as had previously been reported in flies and spiders. Overall, we propose that nub acts downstream of Notch on the distal part of insect leg segments to promote their development and growth, which in turn is required for joint formation. Our data represent the first functional evidence defining a role for nub in leg segmentation and highlight the varying degrees of its involvement in this process across insects.     

Elasticity and movements of the cockroach tarsus in walking

Anatomical, kinematic and ablation studies were performed to evaluate the contribution of elasticity in use of the cockroach tarsus (foot) in walking. The distal tarsus (claws and arolium) engages the substrate during the stance phase of walking by the action of a single muscle, the retractor unguis. Kinematic and ablation studies demonstrated that tarsal disengagement occurs at the end of stance, in part via the action of elastic elements at the penultimate tarsal joint. In isolated legs, this joint exhibits very rapid (less than 20 ms duration) recoil to extension when released from the engaged position, and recoil is even more rapid (less than 10 ms) after removal of the retractor tendon (apodeme). The joint also possesses an enlarged cuticular condyle which is the attachment for ligaments and articular membranes, some of which fulfill morphological criteria consistent with the presence of the elastic protein resilin. Measurements of restoring forces generated by joint displacement indicate that they are graded but could readily lift the mass of the distal tarsus. This biomechanical design can facilitate efficient use of the tarsus in walking while under active control by only a single muscle and may also be highly advantageous when cockroaches very rapidly traverse irregular terrain. Accepted: 16 September 1998     

Environmental and genotype-by-environment influences on chick size in the Yellow-browed leaf warbler Phylloscopus inornatus

Summary I investigated genetic and environmental factors affecting fledgling chick size in the Yellow browed leaf warbler Phylloscopus inornatus. The proportion of variation among broods is >45% for both tarsus length and body weight; this can be attributed at least in part to effects of the shared nest environment. A high off-spring-parent regression for weight also appears to be partly due to an environmentally induced correlation and the regression is reduced when effects of laying date are controlled for. A cross-fostering experiment demonstrated a significant nest of origin x nest of rearing interaction. The presence of genotype-environment interaction affecting chick size may be quite general in birds. It is shown how this can account for observed patterns of the dependence of the magnitude of offspringparent regression on prevailing conditions, as has been found in other studies. In P. inornatus the failure to detect significant genetic variance in chick body weight and tarsus length may be due to high standard errors on the estimates, but it may also reflect true low levels of genetic variance in chick size if the genotype-interaction effect described occurs regularly in this species.