Description
Species of this family are characterized by an elongated head with lateral margins converging backwards; first maxillae with a relatively elongate coxosternite and coxal projections that are much wider than the telopodites, with both appendages ending with a distinctly hyaline part; second maxillae with small simple claws; an elongate forcipular coxosternite with pleurites projecting anteriorly into scapular points and displaced dorsally so that the coxopleural sutures run anteriorly on the dorsal side; metatergites on the posterior part of the trunk that are distinctly longer than those on the anterior part. [1]
This family differs from all other geophilomorphs insofar as the number of segments in mecistocephalids is generally fixed within each species and the same for each sex. [7] [4] The family includes centipedes with odd numbers of leg-bearing segments ranging from as few as 41 to as many as 101. [4] A majority of the species in the genus Mecistocephalus have 49 pairs of legs, [8] a majority so large that most species in the family Mecistocephalidae have 49 leg pairs, [7] even though only one mecistocephalid species ( Proterotaiwanella sculptulata ) outside the genus Mecistocephalus shares this number. [9] [4]
The next most common numbers are 45 leg pairs, found in most species in the genus Tygarrup [6] as well as in species distributed among five other genera, and 41 leg pairs, found in all species in the genera Agnostrup , Anarrup , Arrup , Nannarrup , and Partygarrupius as well as in two species of Dicellophilus ; [10] fewer mecistocephalid species have 47 leg pairs (e.g., Mecistocephalus angusticeps and M. tahitiensis ) or 51 leg pairs (e.g., M. evansi and M. lifuensis ), [4] [11] found only in the genus Mecistocephalus, [9] and the other remaining numbers are even more rare (e.g., 43 leg pairs, found in Dicellophilus carniolensis [10] ). [7]
Intraspecific variation in the number of leg-bearing segments within each sex has been recorded among the mecistocephalid species with the greatest number of legs: Mecistocephalus diversisternus , which has 57 or 59 leg pairs, M. japonicus , which has 63 or 65 leg pairs, and M. microporus , which has odd numbers of leg pairs ranging from 93 to 101. [12] Other mecistocephalid species with many legs are known from samples too small to provide persuasive evidence of intraspecific invariance (e.g., M. cyclops , with 57 leg pairs in the only known specimen). [13] The mecistocephalid species Krateraspis sselivanovi has 53 leg pairs without intraspecific variation, [9] [14] which appears to be the maximum number evidently fixed by species in the class Chilopoda. [15]
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