| abstract | In the parasitic mite family Podapolipidae (Tarsonemina) two phylogenetic trends are examined morphologically and interpreted under aspects of evolutionary biology.
1. The relationsip of the Podapolipid genera is established by the phylogenetic method developed by Henning, and represented in a cladogram (phylogentic tree). The genus Podapolipus appears at the top of a phylogenetic line from which the other genera apparently have split off at various stages of evolution. By its form this cladogram signifies evolution as the result of adapton increasing step by step. A comparison of host ranges shows that the more original genera parasitize exclusvely Coleoptera, whereas the derived ones generally live on Orthoptera. A coevolution of parasite and host is obvious only within certain genera.
2. There are two striking trends in the family Podapolipidae:
a. In the male the copulation apparatus is displaced from the posterior end to the front of the body. In a first step the entire terminal region of hysterosoma, including the 4th pair of legs, is shifted to the dorsal side of the animal. By this process the genital capsule is carried forward so as to reach the posterior margin of the propodosoma. In addition, the genital capsule is elongated so that the copulation apparatus becomes located in front of the gnathosoma. This latter situation has developed independently in two genera.
b. Adult females of different genera form a series with increasing reduction in leg number. The reduction begins with the last pair of legs and proceeds anteriorly until only the 1st pair of legs is retained. In one genus the adult female has no legs at all. The female larvae in all these genera possess thre pairs of legs.
3. All essential stages of these lines of evolution are preserved in living species, wereas normally the better adaption is thought to displace the less suitable. Two reasons for this can be given:
a. Features involved exclusively in sexual functions, such as the copulation apparatus, are probably without significance as far as interspecific competion is concerned, because interspecific competion concerns only factors of the ecological niche.
b. Because of their host specifity, the genera of Podapolipidae live for the most part in strict isolation (comparable to geographic isolation).
4. The displacement of the copulation apparatus and the reduction of legs are morphological adaption likely to have occured subsequent to changes in behaviour (behaviour as pace-maker of evolution):
a. In the family Tarsonemidae (sister group of the Podapolipidae), the actual copulation is introduced by a so-called precopulation with the female larva. From this a genuine copulation with the female larva may have developed in the family Podapolipidae. The change of the mating stage probably gave rise to a new copulation position (change from "retroconjugate" to "proconjugate" position), to which the copulation apparatus was adapted by displacement.
b. If mating takes place already in the female larva, this larva following copulation can serve as migrating stage. Therefore, the adult female can limit its activities to food consumption, growth , and egg production. Thus, legs become superfluous and may disappear; this may even be of slight advantage since the legs might interfere with the physogastric swelling of the body.
The essential evolutionary steps appear to be not changes in patterns of behaviour but changes in timing of previously established patterns, caused by per se insignificant changes of the releasing situations. |