Megaphragma mymaripenne

Megaphragma mymaripenne is a microscopically sized wasp. At 200 μm in length, it is the third smallest extant insect,^[1] comparable in size to single celled organisms. It has a highly reduced nervous system, containing only 7400 neurons, several orders of magnitude fewer than in larger insects. This is the smallest known number of neurons in all insects and in all flying animals. Its average lifespan at adulthood is 5 days.^[2]

In January 1920 specimens of an unknown insect were collected in Hawaii associated with thrips, to which they were suspected to be egg parasites. The insects were described as a new species and genus Megaphragma mymaripenne, in 1924 by Philip Hunter Timberlake. M. mymaripenne specimens were next found on 29 March 1927, again with thrips, on the leaf of a Croton genus plant. On 10 May 1930 specimens were found near thrips and thrip eggs, and mature pupae were found inside the thrip eggs. As of 1930 the genus was not thought to be native to Hawaii.^[3]

Nervous systems are one of the principal factors that limit shrinking body size. The entire central nervous system forms 6 per cent of the body mass of M. mymaripenne, and the brain itself makes up 2.9 per cent. Of the wasp's 7400 neurons, 4600 are located in the brain. Small insects from other families often deal with the issue of having a large brain in relation to their head size by shifting their brain into their thorax and even their abdomen. However, wasps cannot, as to keep their heads flexible the head's connection to the thorax is relatively limited.^[2]

Uniquely, by the time M. mymaripenne reaches adulthood 95 per cent of its nervous cells have lost their nuclei. There are only 339–372 nuclei throughout the central nervous system, of which 179–253 are found in the brain. The nervous system of the pupae of M. mymaripenne makes up 19 per cent of its body mass, 11 per cent of which is the brain. Unlike in adults, cells in the pupae have nuclei. It is only in the final stage of development that these undergo lysis, which greatly reduces the volume of the nervous system. While the brains of pupae are 93,600 μm³, those of the adults are only 52,200 μm³. Accompanying this shrinkage of brain volume is a shrinking of the occipital area of the head, with the cuticle folding into helical spirals.^[2]

Despite their reduced nervous system, adult wasps retain the ability to fly, to feed, and to locate hosts for them to lay their eggs in.^[2] The wasp eggs are deposited in the eggs of thrips.^[1] To emerge, the wasps cut an 80-90 μm near-circular hole in the eggs.^[3]

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