Large Forest-floor Millipedes (Nyssodesmus python, family Platyrhacidae), a member of Flat-backed Millipedes (Order Polydesmida), are typical dwellers of moist and wet forests of the Carribean slope of Costa Rica and are very common in the La Selva Natural Reserve. Every visitor can see them day and night slowly crawling over forest leaf-litter or along foot trails. These are very large millipedes having about 20 segments (each with two pairs of legs) and reaching 10-12 cm in length and up to 2 cm in width. Adults are whitish brown with two dark longitudinal stripes along their backs.Yet they are absolutely harmless. The Millipedes feed on rotten plant materials (detritus, dead leaves, rotten wood) and thus participate in recycling the nutrients of dead organic matter.
An interesting biological feature of these Millipedes is that they are often encountered in pairs, with the smaller male riding on top of the larger female (see below on the photo). This way the male tries to stop the female from mating with other males and to make sure that its own offspring will be produced by the female with which it mated. Copulation may last a few hours. The matter is that copulation does not result in insemination right away. The sperm is stored inside the female until the time of fertilization. Therefore, having mated with a receptive female, the male usually spend up to 5-6 days riding on its back. One can only guess whether it is a leisure time or real work for the rider. Usually the female mates with several males anyway.
When disturbed or frightened, the Millipede rolls up in a ball and can spray toxic liquid containing cyanide on its attacker which can be ejected up to 30 cm. The only protection they have against their enemies.
Henderson C.L. 2010. Butterflies, moths, and other invertebrates of Costa Rica. A field guide. Univ. of Texas Press, Austin, 173 p.