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The masked birch caterpillar, Drepana arcuata (Lepidoptera: Drepanidae) is an excellent model for studying vibratory communication and sociality in larval insects. Vibratory communication occurs throughout development, but the functions of signals are reported to change as larvae change from gregarious to solitary lifestyles. To better understand the sensory ecology of these caterpillars, it is important to study their life history. Here, we describe the morphological and behavioral characteristics of larvae by confirming the number of instars, identifying their distinguishing morphological features, and noting changes in feeding and shelter construction. Five instars were confirmed based on the number of head capsules collected for individuals throughout development, and by using Dyar’s rule, which predicts the number of instars based on geometric growth patterns of head capsules. Frequency distributions of head capsule widths showed five separate peaks, indicating that this is a useful parameter for distinguishing between instars. Other morphological features including body length, shape, and banding patterns of head capsules, and morphology of thoracic verrucae are helpful in distinguishing among instars. Feeding behavior changes from leaf skeletonization in first and second instars to leaf cutting in fourth and fifth instars, with third instars transitioning between these feeding styles as they grow. Early instars typically construct communal silken shelters whereas late instars live solitarily in leaf shelters. These results provide essential life history information on the masked birch caterpillar that will enable future investigations on the proximate and ultimate mechanisms associated with social behavior and communication in larval insects.

The warty birch caterpillar Drepana bilineata produces two distinct types of vibrational signals (mandible drumming and anal scraping) during interactions with conspecifics. Vibrational signalling is characterized using standard and high- speed videography synchronized with laser-doppler vibrometry, and behavioural experiments test the hypothesis that signalling functions to advertise occupancy of birch ( Betula ) leaves. Drumming involves raising the head and striking the leaf with the sharp edges of the open mandibles. Anal scraping involves dragging a pair of specialized oar-shaped setae against the leaf surface. Staged encounters between leaf residents and conspecific intruders result in the resident signalling, with rates increasing as the intruder moves closer. Intruders signal significantly less often than residents. Conflicts are typically resolved within a few minutes, with the resident winning in 61% of the trials, and the intruder winning in 6%. Contests that last more than 30 min are deemed 'ties' and comprise the remaining 33% of trials. The results support the hypothesis that vibrational signals function to advertise leaf occupancy. Vibrational communication is believed to be widespread in Drepanoidea caterpillars, but has only been described in two species to date: D. bilineata (present study) and Drepana arcuata . It is proposed that differences in territorial behaviour and signalling between these species are related to their relative investments in silk leaf mats and shelters. The proximate and ultimate bases for the evolution of vibra- tional communication in caterpillars are discussed.

Leaf-borne vibrations are potentially important to caterpillars for communication and risk assessment. Yet, little is known about the vibratory environment of caterpillars, or how they detect and discriminate between vibrations from relevant and non-relevant sources. We measured the vibratory ‘landscape’ of the territorial masked birch caterpillar Drepana arcuata (Drepanidae), and assessed its ability to detect and respond to vibrations generated by conspecific and predatory intruders, wind and rain. Residents of leaf shelters were shown to respond to low amplitude vibrations generated by a crawling conspecific intruder, since removal of the vibrations through leaf incision prevented the resident’s response. Residents did not respond to large amplitude, low frequency disturbances caused by wind and rain alone, but did respond to approaching conspecifics under windy conditions, indicating an ability to discriminate between these sources. Residents also responded differently in the presence of vibrations generated by approaching predators (Podisus) and conspecifics. An analysis of vibration characteristics suggests that despite significant overlap between vibrations from different sources, there are differences in frequency and amplitude characteristics that caterpillars may use to discriminate between sources. Caterpillars live in a vibration-rich environment that we argue forms a prominent part of the sensory world of substrate bound holometabolous larvae.

We provide evidence for conspecific acoustic communication in caterpillars. Larvae of the common hook-tip moth, Drepana arcuata (Drepanoidea), defend silk nest sites from conspecifics by using ritualized acoustic displays. Sounds are produced by drumming the mandibles and scraping the mandibles and specialized anal “oars” against the leaf surface. Staged interactions between a resident and intruder resulted in escalated acoustic “duels” that were typically resolved within minutes, but sometimes extended for several hours. Resident caterpillars generally won territorial disputes, regardless of whether they had built the nest, but relatively large intruders occasionally displaced residents from their nests. All evidence is consistent with acoustic signaling serving a territorial function. As with many vertebrates, ritualized signaling appears to allow contestants to resolve contests without physical harm. Comparative evidence indicates that larval acoustic signaling may be widespread throughout the Lepidoptera, meriting consideration as a principal mode of communication for this important group of insects.