Crickets are small, omnivorous insects found all over the world, including in Hong Kong. Strictly speaking, ‘true crickets’ only refer to members of the family Gryllidae, but many of their close relatives are inopportunely similarly named—including the African mole cricket (𝘎𝘳𝘺𝘭𝘭𝘰𝘵𝘢𝘭𝘱𝘢 𝘢𝘧𝘳𝘪𝘤𝘢𝘯𝘢) shown here. Along with grasshoppers, locusts and katydids, crickets are part of the insect order Orthoptera, many of which can use their powerful hind legs to jump long distances . The African mole cricket, however, spends most of its time underground, and has strong, shovel-shaped front legs adapted for digging burrows. Adults are about 3-4 cm long, and feed on a variety of plants and other insects. Although they are native to Africa, these mole crickets have been introduced to many other parts of the world, where they are sometimes considered agricultural pests.
“Chirp! Chirp!” In the summer, you may hear this loud noise coming from the bushes. These are in fact calls of male crickets, trying to attract mates. Like birds, each species has its unique call, so a trained pair of ears can identify them just by listening carefully! Similar to most insects, crickets are less active at lower temperatures, and their calls become slower. In fact, scientists have even discovered a way to estimate the ambient air temperature based on the chirping rate of the snowy tree cricket (𝘖𝘦𝘤𝘢𝘯𝘵𝘩𝘶𝘴 𝘧𝘶𝘭𝘵𝘰𝘯𝘪). According to Dolbear’s law, temperature (°F) = 40 + number of chirps in 15 seconds. Maybe you could also try this the next time you hear crickets chirping!
So how do male crickets create this loud calling sound? Like other orthopterans, the forewings of crickets are modified into tough, leathery structures known as tegmina (singular tegmen). Stridulations, or the repeated striking of the edge of one tegmen against the other, create vibrations. These vibrations are then converted into sound by an area of thick membrane on the tegmen known as the harp . 𝘎𝘳𝘺𝘭𝘭𝘰𝘵𝘢𝘭𝘱𝘢 can further amplify their singing using the natural resonance of their horn-shaped burrows, producing sounds that can be as loud as a motorcycle (115 dB) near the entrance ! Females, however, lack the necessary structures to create stridulations and therefore cannot chirp; but they can hear and respond to male calls using the simple “ears” on their front legs—a membrane (the tympanum) stretched across an air-filled tube. Yes, some Orthoptera have “ears” on their legs, something you can observe in nature.
Crickets have many natural predators, as they are an important food source for spiders, lizards, birds, and, in the case of the African mole cricket, humans too! But one of their most fascinating enemies is a type of parasite known as the horsehair or Gordian worm (Phylum: Nematomorpha). Larvae of these worms are transported out of water via an intermediate host (e.g., a mosquito), which gets eaten by the cricket on land. The worms then mature within the crickets’ bodies; and once they reach the adult stage, begin to manipulate the behaviour of their cricket host, causing it to move erratically and eventually jump into a body of water . While the cricket drowns, the long and slender adult worms then slither out of its body to reproduce and continue the life cycle of the parasite. Remarkably, the horsehair worms can even escape from a predator (e.g., a fish or frog) that has eaten the dying cricket, by wriggling out of their nose, mouth or gills!
As you can see, like all insects, crickets are not only fascinating in their own right, but also form many interesting interactions with other organisms. To learn more about their wonderful biology and of other organisms, pay us a visit at the Hong Kong Biodiversity Museum!
Text: Jasper Lee
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