The Spotted Predatory Katydid (Chlorobalius leucoviridis) is a large, cryptically colored, green-and-white katydid in the subfamily Listroscelidinae. It is found throughout the arid interior of Australia. Adults are active in the summer and can be found in the tops of large shrubs and small trees during both day and night. Both males and females have file-and-scraper structures on the forewings (tegmina), and males make loud irregularly broken trilling songs at night to attract conspecific (i.e., same species) females. Female acoustic behavior has not yet been studied, but it is likely that female C. leucoviridis silently approach a calling male and mate. (Marshall and Hill 2009)
Marshall and Hill (2009) reported their discovery that C. leucoviridis can attract male cicadas of the tribe Cicadettini (Hemiptera: Cicadidae) by imitating the species-specific wing-flick replies of sexually receptive female cicadas. This aggressive mimicry is accomplished both acoustically, with tegminal clicks, and visually, with synchronized body jerks. (The term "aggressive mimicry" is used to describe the phenomenon by which a predator or parasite imitates a signal of another species in order to exploit the recipient of the signal.) Remarkably, C. leucoviridis katydids respond effectively to a variety of complex, species-specific cicada songs, including songs of many cicada species they cannot ever have encountered. Most cicadettine cicada species exhibit stereotyped signal-response “duets” during sexual pair-formation. Males sing a species-specific calling song containing a particular song element that triggers wing-flick responses from nearby sexually receptive female cicadas. The female responses are simple, brief (1 ms), broad-frequency sounds, and they are sometimes audible from many meters away. Because a wing-flick reply is structurally nondescript, it must closely follow the cue in the male cicada's song in order to be recognized. Males locate females by listening for these responses and by searching visually for wing-flicking females when at close range. Recordings of cicada songs with reply clicks from C. leucoviridis katydids can be heard here. Sonograms of a C. leucoviridis calling song, male-female duets for two cicada species, and male C. leucoviridis deceptively duetting with various cicada species can be seen here (several video clips of C. leucoviridis males calling and attracting and capturing male cicadas can be seen here as well). (Marshall and Hill 2009)
Marshall and Hill (2009) carried out experiments to document and investigate this striking example of aggressive mimicry. In a typical experimental trial, soon after the cicada began to sing, one or more katydids began responding, with many clicks following male song cues. The male cicada then turned toward and began walking and/or flying toward a replying katydid while continuing to sing, just as the authors observed occurred in cicada pair-formation. Once the cicada came within reach, the katydid grabbed it with its forelegs (and often midlegs) and subdued it by partially biting off the head, a behavior common to many predatory orthopterans. Marshall and Hill report that in their observations captured cicada prey were always held by the katydids between the base of the tarsi, with the tarsal claws held away from the prey. The katydids often hung by only their hind legs to consume and sometimes to catch their prey. The formidable spines on their legs did not appear to come into contact with prey items. Captured cicadas were entirely consumed except for the forewings. (Marshall and Hill 2009)
Marshall and Hill report that in most of their observations the replying C. leucoviridis bounced or jerked their bodies precisely in time with each click reply (see video here). They note that tegminal movements made during regular male katydid song did not cause similar incidental body movements, suggesting that these body movements are a form of visual mimicry.
Observations suggested that C. leucoviridis did not reply to cicada song in darkness, despite the fact that the katydids are active and sing at night. Marshall and Hill found that male C. leucoviridis would often begin singing as soon as it became dark, but seemed to switch from a “predatory” mode into a “courtship” mode with a decrease in light levels. They also noted that they never heard caged C. leucoviridis responding to sharp ambient sounds at night, although this frequently happens in daylight. Cicada prey are highly diurnal, while most C. leucoviridis male singing activity and flight occurs at night or at dusk. Marshall and Hill suggest that the experimental finding that the C. leucoviridis click mechanism operates only during daylight is expected if the click response has evolved as a tool for capturing cicadas, but would not be expected if the behavior has evolved in the context of intraspecific communication. In addition to variation in responsiveness associated with light levels, individual C. leucoviridis demonstrated substantially different levels of overall responsiveness to cicada songs and these “personalities” remained stable over many weeks of observations. All C. leucoviridis that were captured because they were found clicking in reply to cicada songs in the field were consistently responsive to cicada songs and other sharp noises. In contrast, of the males that were captured while singing, one was highly responsive while the other seven (and the two females) were much less so. (Marshall and Hill 2009)
Marshall and Hill (2009) discuss various evolutionary implications and possibilities emerging from their initial studies of aggressive mimicry of cicadas by C. leucoviridis.
Rentz (1995) investigated the possible role of the long spines on the fore- and midlegs in predatory behavior by C. leucoviridis capturing flies, grasshoppers, and other katydids. In lab trials, Rentz found that prey were captured in the same way nearly every time: a prey item would be bitten under the throat and immobilized almost immediately. Whether or not the spines on the forelegs and midlegs were used seemed possibly to depend on the size of the prey item. In consuming prey, some of the spines of the middle femur appeared to serve to stabilize prey.
