# Mecistocephalus angusticeps

Mecistocephalus angusticeps is species of soil centipede in the family Mecistocephalidae. This centipede is found in Kenya, Seychelles, and the Chagos Archipelago. This species features only 47 pairs of legs rather than the 49 leg pairs usually observed in the genus Mecistocephalus.

## Taxonomy and distribution

The French zoologist Henri Ribaut first described this species in 1914 based on a single adult female specimen found in 1911 in one of the Shimoni caves at sea level on the coast of Kenya. Ribaut originally described this species under the name Lamnonyx angusticeps. In 1920, the American biologist Ralph V. Chamberlain moved this species to the genus Mecistocephalus. Since then, this species has been found on several islands in the Indian Ocean. Finds have been recorded on islands in Seychelles (at Baie Laraie on Curieuse island, on Picard island and elsewhere on the Aldabra atoll, and at Fond de l'Anse on Praslin island) and on the Egmont Atoll in the Chagos Archipelago.

## Phylogeny

A phylogenetic analysis of the family Mecistocephalidae based on morphology places this species in a clade with two other Mecistocephalus species with only 47 pairs of legs, M. tahitiensis and an undescribed species found on the Marquesas islands in French Polynesia. This analysis places a clade with only 45 leg pairs in the most basal branch of a phylogenetic tree of the genus Mecistocephalus, with a sister group formed by species in this genus with 47 or more leg pairs, and the clade with only 47 leg pairs in the second most basal branch, with a sister group formed by species in this genus with 49 or more leg pairs. This analysis indicates that the common ancestor of the species in this genus had 45 leg pairs, and that species with more leg pairs evolved through a process that added segments and increased the number of legs incrementally, first to 47 pairs, then (for most species in the genus) to 49 pairs.

## Description

This species has only 47 pairs of legs and reaches 50 mm in length. The body is mostly yellow, with fine dark pigmentation on the dorsal surface, but the head and most anterior segments are a reddish brown. The head is longer than wide, with a length/width ratio of about 1.7 or 1.8, and the antennae are three times as long as the head is wide. The second maxillae are slender, each with a claw that tapers uniformly to a pointed tip. The first article of the forcipules features a small proximal tubercle and a similar small distal tubercle, the intermediate articles each feature a small tubercle, and the ultimate article features only a shallow emergence at the base. The sternum of the last leg-bearing segment has a trapezoidal shape and is more wide than long, with a notch on each lateral margin.
This species shares many features with other species in the genus Mecistocephalus. For example, the head in this species is evidently longer than wide. Furthermore, each of the second maxillae in this species ends in a claw. Unlike most other species in this genus, however, this species features only 47 leg pairs rather than the 49 pairs usually found in this genus.
Although both this species and its close relative M. tahitiensis have 47 leg pairs, other features distinguish these two species. For example, the second maxillae in M. tahitiensis are unusually short, significantly shorter than in M. angusticeps. In M. tahitiensis, the second maxillae do not extend beyond the first maxillae, whereas in M. angusticeps, the second maxillae are of normal size for a species of Mecistocephalus, reaching beyond the first maxillae.