# Mecistocephalus nannocornis

Mecistocephalus nannocornis is a species of soil centipede in the Mecistocephalidae family. This centipede has only 45 pairs of legs, the minimum number recorded in the genus Mecistocephalus. This centipede was the second species in this genus to be discovered with such a modest number of legs.

## Discovery and taxonomy

This species was first described in 1920 by the American biologist Ralph V. Chamberlin. He based the original description of this species on a female holotype found by the American biologist Charles F. Baker on Mount Makiling on the island of Luzon in the Philippines. This holotype is deposited in the Museum of Comparative Zoology at Harvard University.
In 1961, the Japanese myriapodologist Keizaburo Shinohara described M. manazurensis as a new species in the same genus. Shinohara based this description on specimens collected from the town of Manazuru in the Kanagawa prefecture on the island of Honshu in Japan. In 2007, the Italian biologists Marco Uliana, Lucio Bonato, and Alessandro Minelli deemed M. manazurensis to be a junior synonym of M. nannocornis. Some authorities consider these centipedes to be the same species, but other authorities continue to list both as valid species, although some also acknowledge that these centipedes are probably the same species.

## Distribution

This centipede is found not only in the Philippines but also in Taiwan and Singapore, where this species has been recorded in the Bukit Timah area. Including M. manazurensis as a junior synonym, M. nannocornis is also found on the island of Honshu in Japan.

## Phylogeny

A phylogenetic analysis of the family Mecistocephalidae based on morphology places M. nannocornis in a clade with M. spissus, which was the first species in the genus Mecistocephalus to be discovered with only 45 leg pairs. This analysis also places this clade on the most basal branch of a phylogenetic tree of this genus, with a sister group formed by all the other species in this genus together in a separate clade. This analysis suggests that the common ancestor of the centipedes in this genus had 45 leg pairs and that the species in this sister group evolved through a process that added segments and increased the number of legs.

## Description

This species is orange with dark patches in at least some specimens. This centipede has 45 leg pairs and can reach 27 mm in length. The head is longer than wide, with a length/width ratio of 1.5. The antennae are about 2.5 times longer than the head is wide. Each of the second maxillae ends in a claw. The sternum of the last leg-bearing segment is shaped like a trapezoid that is slightly wider than long, with a width/length ratio of 1.1. The basal element of each ultimate leg features numerous pores of moderate size.
This species shares many traits with other species in the genus Mecistocephalus. For example, each of the second maxillae in this species features an apical claw. Furthermore, the head in this species is evidently longer than wide, and a spiculum projects from each side of the head. Unlike most other species in this genus, however, this species features only 45 leg pairs rather than the 49 pairs usually found in this genus.
Although both this species and its close relative M. spissus have only 45 leg pairs, there are also clear differences. For example, M. spissus features a greater number of teeth on the four articles of the forcipules, with two on the first article, one on the second, one on the third, and two (one ventral and one dorsal) on the fourth. The species M. nannocornis has only one distal tooth on the first article, a tubercle on the second, one tooth on the third, and no distinct tooth on the fourth. Furthermore, M. spissus has a more elongated head, with a length/width ratio of 1.7 to 1.8, whereas M. nannocornis has a notably short head, with a length/width ratio of only 1.5.