Male-specific leg modifications in five species of European millipedes (Diplopoda, Julida)

Millipedes (Diplopoda) are sexually dimorphic in various morphological, behavioral and post-embryonic development traits. Morphological intersexual variation in members of the diplopod order Julida encompasses the presence of gonopods (male specialized appendages that are involved in sperm transfer), the presence of glands that open on the coxae of the second leg-pair and modifications of gnathochilarium, mandibles and legs. In order to examine fine morphology of the latter sexually dimorphic trait in five julidan species [Cylindroiulus boleti (C. L. Koch, 1847), Megaphyllum bosniense (Verhoeff, 1897), M. unilineatum (C. L. Koch, 1838), Ommatoiulus sabulosus (Linnaeus, 1758) and Pachyiulus cattarensis (Latzel, 1884)], we dissected anterior (including the first) leg-pairs in males of the mentioned species and examined them using light and scanning electron microscopy. Our results show that the first leg-pair in all analyzed species shares common features: the legs are reduced in size (compared to other walking legs), consist of only three segments, and are hook-like. The other legs are characterized by the presence of adhesive pads on the ventral surface of the tibia and tarsi. The adhesive pads are of a ‘smooth’ type: they are made of a thin layer of cuticle with only a few striations. Since these modifications of the walking legs are present only in males of the analyzed species, we propose that these structures enable males to achieve a firmer grip of the female head (hook-like legs of the first pair) and to hold onto the female during mating (walking legs with adhesive pads).