ARACHNIDA 537 



and behind the 4th legs (Metastigmata). If we regard the opistho- 

 somatic position of the breathing organs as primitive it is difficult to 

 see how these varying arrangements have come to pass in the 

 acarines. 



The life history of the parasitic forms is of great interest, especially 

 that of the ticks or Metastigmata. These are divided into the Ixodidae 

 (Fig. 365 B) and Argasidae. The former live permanently on one host; 

 the life of Boophilus bovis, attached to the cow, is only interrupted 

 by the necessity of moulting and reproduction. Though compelled to 

 withdraw its mouth parts when the skin is being cast, the tick plunges 

 them into the host again at the same place, as soon as possible after the 

 completion of the process. In many other cases the ticks fall off before 

 every moult and have to seek a new host afterwards. The argasids, 

 however, in the full-grown state, make only short visits to the host to 

 suck blood, lasting for a few hours. In these last cases the young can 

 go without food for months and the full-grown tick for years. In the 

 course of several of these meals the six-legged larva develops into an 

 eight-legged nymph which becomes sexually mature only after further 

 development. Copulation may take place several times, spermato- 

 phores being inserted, but the sperm in these can only escape and 

 reach the ovary after the female again feeds. But in all cases when 

 fertilization of the eggs has once occurred, the female falls to the 

 ground and after laying her eggs dies. 



Many kinds of ticks carry disease, e.g. in both the following cases 

 caused by Spirochaeta, Texas fever of cattle (Boophilus annulatus) 

 and the relapsing fever of man (Ornithodorus moubata). Also certain 

 small parasites of the blood corpuscles {Piroplasma), in severe diseases 

 of cattle, are carried largely by Rhipicephalus . 



Class PHALANGIDA 



Arachnids with prosoma covered by a single tergal shield and united 

 to the opisthosoma by its whole breadth; opisthosoma always seg- 

 mented ; chelicerae three-jointed and chelate ; pedipalps leg-like ; two 

 simple eyes. 



These creatures, with their enormous elongated legs, are familiar 

 objects in the summer; the active predaceous forms are supposed to 

 live for a single season only, but some representatives are slow-moving 

 and live longer. They feed on insects and other arthropods and suck 

 their juices. The walking legs have the same number of joints as^ 

 spiders, but the tarsus is multiarticulate. The opisthosoma contains at 

 least ten segments. The animal breathes by tracheae and there are 

 two stigmata on the first sternum of the opisthosoma, opening on each 

 side of the reproductive aperture from which emerges a long pro- 



