Long Frontal Projections Help Battus philenor (Lepidoptera: Papilionidae) Larvae Find Host Plants

Abstract

Animals sometimes develop conspicuous projections on or near their heads as, e.g., weaponry, burrowing or digging tools, and probes to search for resources. The frontal projections that insects generally use to locate and assess resources are segmented appendages, including antennae, maxillary palps, and labial palps. There is no evidence to date that arthropods, including insects, use projections other than true segmental appendages to locate food. In this regard, it is noteworthy that some butterfly larvae possess a pair of long antenna-like projections on or near their heads. To date, the function of these projections has not been established. Larvae of pipevine swallowtail butterflies Battus philenor (Papilionidae) have a pair of long frontal fleshy projections that, like insect antennae generally, can be actively moved. In this study, we evaluated the possible function of this pair of long moveable frontal projections. In laboratory assays, both frontal projections and lateral ocelli were shown to increase the frequency with which search larvae found plants. The frontal projections increased finding of host and non-host plants equally, suggesting that frontal projections do not detect host-specific chemical cues. Detailed SEM study showed that putative mechanosensillae are distributed all around the frontal as well as other projections. Taken together, our findings suggest that the frontal projections and associated mechanosensillae act as vertical object detectors to obtain tactile information that, together with visual information from lateral ocelli and presumably chemical information from antennae and mouthparts, help larvae to find host plants. Field observations indicate that host plants are small and scattered in southern Arizona locations. Larvae must therefore find multiple host plants to complete development and face significant challenges in doing so. The frontal projections may thus be an adaptation for finding a scarce resource before starving to death. This is the first evidence that arthropods use projections other than true segmental appendages such as antennae, mouthparts and legs, to locate food resources.

Long horns protect Hestina japonica butterfly larvae from their natural enemies

“…To the best of our knowledge, larval horns seem to have evolved repeatedly within the Nymphalidae and nowhere else in the Lepidopteran phylogeny. By studying additional butterfly species, we are currently testing the hypothesis that the former type of projection may be widely used for host plant search, as shown for B. philenor 27 , and the latter for defence against natural enemies, as shown in this study for H. japonica .…”

“…The function of these projections is generally not well known. One exception is the use of projections by larvae of the pipevine swallowtail Battus philenor to locate and assess food 27 . Larvae of Hestina japonica (Nymphalidae) also have a pair of long projections.…”

Adaptive ontogenetic shifts in larval responses to environmental cues

“…Caterpillars perceive light through stemmata (simple eyes) located on the head (Gilbert, 1994;Stehr, 2009b;Chapman, 2012), as well as extraocular photoreceptors located in the skin (Eacock et al, 2019). Color and light intensity can be used by larvae to locate a host plant (Kandori et al, 2015), or by prepupae to locate a pupation site (Wiklund, 1972;Roden et al, 1992;Veldtman et al, 2007). Some species display developmental plasticity during pupation when exposed to various light intensities or colors (Hazel & West, 1996;Hazel et al, 1998).…”

Mouth morphology of the diving beetle Hyphydrus japonicus (Dytiscidae: Hydroporinae) is specialized for predation on seed shrimps

“…In addition to defence, horns are used for feeding. Larvae of the pipevine swallowtail Battus philenor L. (Papilionidae) have long fleshy frontal projections that help them find host plants, although these projections develop on the prothorax (Kandori et al, 2015). As predatory insect larvae and nymphs generally develop large mandibles, labium or claws (e.g.…”