Insects have evolved a great diversity of sound-producing mechanisms largely attributable to their hardened exoskeleton, which can be rubbed, vibrated or tapped against different substrates to produce acoustic signals. However, sound production by forced air, while common in vertebrates, is poorly understood in insects. We report on a caterpillar that 'vocalizes' by forcing air into and out of its gut. When disturbed, larvae of the Nessus sphinx hawkmoth (Sphingidae: ) produce sound trains comprising a stereotyped pattern of long (370 ms) followed by multiple short-duration (23 ms) units. Sounds are emitted from the oral cavity, as confirmed by close-up videos and comparing sound amplitudes at different body regions. Numerical models using measurements of the caterpillar foregut were constructed to test hypotheses explaining sound production. We propose that sound is generated by ring vortices created as air flows through the orifice between two foregut chambers (crop and oesophagus), a mechanism analogous to a whistling kettle. As air flows past the orifice, certain sound frequencies are amplified by a Helmholtz resonator effect of the oesophagus chamber. Long sound units occur during inflation, while short sound units occur during deflation. Several other insects have been reported to produce sounds by forced air, but the aeroacoustic mechanisms of such sounds remain elusive. Our results provide evidence for this mechanism by showing that caterpillars employ mechanisms similar to rocket engines to produce sounds.

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Paranastatus Masi, 1917 (Eupelmidae, Eupelminae) was originally described based on two species from Seychelles: Paranastatus egregius and Paranastatus violaceus. Eady (1956) subsequently described Paranastatus nigriscutellatus and Paranastatus verticalis from Fiji. Here, four new species of Paranastatus are described: Paranastatus bellus Scallion, sp. n. and Paranastatus pilosus Scallion, sp. n. from Indonesia, and Paranastatus halko Scallion, sp. n. and Paranastatus parkeri Scallion, sp. n. from Fiji. A key to all Paranastatus species based on females is included and lectotypes are designated for Paranastatus egregius and Paranastatus violaceus. Finally, previously unobserved colour variation from newly collected material of Paranastatus verticalis, distribution patterns of species, and possibilities for future research are discussed.