Mechanism Of Vocalisation Of Sound Production In Juveniles
Sound production in the Lepidoptera are reported to occur during juvenile and adult life stages through variety of different mechanisms, depending on species. A recent comparative analysis has identified four distinct types of sound production mechanisms for Bombycoidea larvae, 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 byproduct 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. Until recently, sound production by airflow has only been extensively studied and experimental confirmed within the literature for two species of insect, the Death’s-head hawkmoth (Acherontia atropos) and the Madagascar hissing cockroach (Gromphadorhina portentosa).
Vocalisation has been reported for several species of Bombycoidea caterpillar, such as Sphecodina abbottii, Amphion floridensis, Pachygonidia drucei and Nyceryx magna, Amorpha juglandis. 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 was shown to force air into and out of the gut when attacked by birds, which generates a whistling noise. Sound is produced by ring vortices generated as air flows through the narrow opening between the oesophagous and crop chambers. Other recent studies have shown that the North American walnut sphinx (Amorpha juglandis) produce trains of high pitched whistles by expelling air through their spiracles (external respiratory opening) when attacked by an avian predator.
Vocalisation as a mechanism of sound production in juveniles remains poorly understood. Although recent studies have advanced current understanding of vocalisation in juvenile life stages, further research is required. Only very few species of the Sphingidae family are known to vocalise as adults as part of a defence strategy. Until recently, squeaking (vocalization) as a defence mechanism was only documented for Langia zenzeroides, Pseudoclanis postica, and Pseudoclanis molitor. Vocalisation is now documented in all extant species during adult life stages in the Death's-head hawkmoth genus Acherontia. For example, the African death's head hawkmoth (Acherontia 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.
In juveniles, 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 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. It is well known that the production of ultrasonic clicks by moths serve as an important anti-bat strategy.
The defensive role of ultrasonic clicks against bat predation is widespread across moths from the former lepidopteran family Arctiidae (now Erebidae) within the superfamily Noctuoidea. 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 garden tiger moth (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 ruby tiger moth (Phragmatobia fuliginosa) and the dogbane tiger moth (Cycnia tenera) have striated tymbal organs containing a number of ruby, which produce a train of clicks for each tymbal buckling; one click for each microtymbal. Recent studies have found that hawkmoths in the Choerocampina emit ultrasound using genital stridulatory structures.
Although the function remains unknown, it is believed that these clicks are used as a predator defense 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.
