Sound Production Type During Defence in The Lepidoptera

Sound production in the Lepidoptera is reported in adults and larvae through a variety of different mechanisms, depending on species. A recent comparative analysis has identified four distinct types of sound production mechanisms for caterpillars from the Bombycoidea superfamily, including whistling, chirping, clicking and vocalising.

Vocalisation, such as squeaks and hissing, is an unusual and under documented form of sound production for insects. Vocalisation has been defined as any sound produced as a by-product of eating or through the action of an animal's’ respiratory system (airflow). These sounds can be produced either through aerodynamic mechanisms or by a mechanically vibrating element. Until recently, vocalisation was undocumented for caterpillars. Initially, sound production using airflow mechanisms was only extensively studied and experimental confirmed for two species of insect, the African Death’s-head hawkmoth (Acherontia atropos), and the Madagascar hissing cockroach (Gromphadorhina portentosa). Vocalisation is now observed for several caterpillars from the subfamily Macroglossinae. For instance, vocalisation is reported for Sphecodina abbottii, Amphion floridensis, Pachygonidia drucei and Nyceryx magna. However, the proposed mechanisms of sound production for these species was solely based on observations that the mandibles (chewing parts) were held in the open position during sound production and remains unverified experimentally. A recent study has provided experimental evidence to support vocalisation in the Nessus sphinx hawkmoth A. floridensis larvae. A. floridensis larvae force air into and out of the gut when attacked by birds, emitting a whistling noise through the oral cavity.

Sound is produced by ring vortices created as larvae force air through the narrow opening between the crop and oesophagus (foregut chambers). Although recent studies have advanced current understanding of vocalisation in some larval Lepidoptera, further research is required. Unlike vocalisation, whistling is generated by airflow through the abdominal spiracles (external respiratory opening of insects). Defensive whistling is only observed in caterpillars in the subfamily Smerinthinae. The North American walnut sphinx (Amorpha juglandis) produce trains of high pitched whistles by expelling air through two enlarged spiracles on the eighth abdominal segment when attacked by avian predators. A similar mechanism is documented for the hornworm caterpillar (Langia zenzeroides). Defensive whistle observed in these two species is an example of convergent evolution, since Amorpha and Langia are not closely related.

In larval Lepidoptera, clicks and chirps are the most common form of sound production and is typically generated through stridulation (the act of producing sound by rubbing or striking special bodily structures). Chirps are generated by rubbing the ridged ‘teeth’ edge of one mandible against the smooth interior surface of the other, which typically produce a train of pulses. Short clicks are produced by snapping the anterior side of one mandible against the serrated inner surface of the opposing mandible, whereas other species are shown to produce a train of clicks using the serrated ‘teeth’ on opposite mandibles. Very few species of the Sphingidae family are known to vocalise as adults as part of a defence strategy. Until recently, squeaking as a defence mechanism was only documented for hornworm moth L. zenzeroides, the mulberry hawk moth Pseudoclanis postica, and the African Pseudoclanis molitor. Vocalisation is now documented in all extant species in the Death's-head hawkmoth genus Acherontia. For instance, A. atropos will emit a series of high pitched squeaks if irritated, which is produced by the opening and closing of the epipharyngeal lobe. It is believed that certain evolutionary morphological traits specific to Acherontia have facilitated pharyngeal sound production, such as the modified and enlarged epipharynx and shortening of the proboscis, which evolved to increase viscous honey uptake efficiency. It is emphasised that the production of ultrasonic clicks by moths serves as an important anti-bat strategy.

The use of ultrasonic clicks is widespread in tiger moths. The type of clicks depends on the morphology of the sound producing tymbal organ, which is found in the abdomen of moths. For instance, some species, such as the the garden tiger moths (Arctia caja) have smooth tymbal organs and produce two sharp clicks for each tymbal buckling; one for the inward, and another for the outward buckling. Other species, such as the dogbane tiger moth (Cycnia tenera) have striated tymbal organs containing a number of microtymbals, which produce a train of clicks for each tymbal buckling; one click for each microtymbal.

Recently, it is discovered that adult hawkmoth (family Sphingidae) emit ultrasound using genital stridulatory structures when stimulated by bat sonar attack. Although the function remains unknown, it is speculated that these clicks are used as a predator defence against bats since sound production by stridulation is elicited when exposed to playback of bat sonar. The majority of information on the clicking behaviour of moths as an anti-predator strategy appears to come from tactile or acoustic stimulation experiments. Very little data is collected under field conditions.