Mecistocephalus spissus

Mecistocephalus spissus is a species of centipede in the Mecistocephalidae family. The American biologist Horatio Curtis Wood first described this species in 1862 based on type material found in Kauai or Oahu in Hawaii. This centipede has only 45 pairs of legs, the minimum number recorded in the genus Mecistocephalus, and was the first species in this genus to be discovered with such a modest number of legs.

Distribution
The species is endemic to the Hawaiian Islands. This centipede is found on most of the major islands, mainly in mountainous areas. Published records report finds on Hawaii, Kauai, Maui, Molokai, Necker, and Oahu.

Phylogeny
A phylogenetic analysis of the family Mecistocephalidae based on morphology places this species in a clade with M. nannocornis, which was the second 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 dark yellow-brown with no dark patches, has 45 leg pairs, and can reach 70 mm in length. The head has a trapezoidal shape, and both the head and body taper toward the posterior end. The forcipules are stout, and when closed, extend beyond the anterior margin of the head. The sternum of the ultimate leg-bearing segment has the shape of a shield, and the ultimate legs are slender with scattered setae and no sexual dimorphism.

Although both this species and its close relative M. nannocornis 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 head with a length/width ratio of 1.7 to 1.8, whereas M. nannocornis has a head that is unusually short, with a length/width ratio of only 1.5.