Polionemobius taprobanensis

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Polionemobius taprobanensis is a ground cricket found across Asia. A juvenile has been observed in Bentota, Sri Lanka.

Taxonomy and history
It belongs to the ground cricket family Trigonidiidae. The species was first described in 1869 by Francis Walker as Trigonidium taprobanense from a Sri Lankan specimen.

Two forms
There are two forms: subtropical (southern) and temperate (northern). A 2023 study of mitochondrial genomes suggests the temperate form found in Japan, as well as populations from China and other parts of mainland Asia, may be different enough to be a separate species.

Subtropical female description (simplified)
The subtropical form is very dark (nearly pitch-black). The head has three stripes and the face is brick-red. The eyes are large; the prothorax has spots on the sides. The oviduct is nearly as long as the abdomen. Hind legs have three long spines and are banded. Forewings are ash-grey with seven brown dots; hind wings are clear and about twice as long as the fore wings. The body is about 4.2 mm long.

Distribution
This species ranges from Sri Lanka and Java to the Russian Far East. In Japan, there are two forms: the temperate form on the main islands (formerly called P. mikado) and the subtropical form in the Ryukyu Islands. Their ranges are roughly divided by the archipelago.

Life cycle note
The temperate form shows photoperiod-dependent egg diapause (eggs pause development depending on day length), a trait not usually found in the subtropical form. Hybrids between the two forms can be fertile.