356 TICKS 



they undergo internal reorganization. Finally they shed their 

 skins and emerge as eight-legged but sexually immature ticks 

 known as nymphs (Fig. 157C). The nymphs climb up on bushes 

 or weeds and again there is a period of patient waiting, resulting 

 either in starvation or a second period of feasting. Once more 

 the ticks drop to the ground to rest, transform and moult, this 

 time becoming fully adult and sexually mature. In this condition 

 a host is awaited for a third and last time, copulation takes place, 

 sometimes even before a final host is reached, and the females 

 begin their final gluttonous feeding which results in distending 

 them out of all proportions. In some species, especially those 

 which live on hosts which return to fixed lairs, copulation takes 

 place off the host. When this occurs, as in many species of 

 Ixodes, the male is often not parasitic at all, and may differ 

 markedly from the female in the reduced structure of its hypo- 

 stome (Fig. 152C, E and F). In all species the males die shortly 

 after copulation. 



This, in general, is the life history of ticks, but it is, of course, 

 subject to considerable variation in different species. In many 

 species there are two nymphal periods instead of one. In some 

 species, as in the Texas fever tick, Margaropus annulatus, the 

 moulting takes place directly on the host, thus doing away with 

 the great risk of being unable to find a new host after each suc- 

 cessive moult. In a few species the first moult is passed through 

 on the host, but the second is passed on the ground. The most 

 important asset of ticks to counterbalance the disadvantage of 

 having to find new hosts is their extraordinary longevity. Larvae 

 of ticks have been known to live more than six months without 

 food, and adults have been kept alive in corked vials for five 

 years. 



There are two families of ticks, the Argasidae and the Ixodidae. 

 The Argasidae include the bird ticks and their allies, which are 

 distinguished from the Ixodidae by the absence of a dorsal shield 

 and in having the head partially or entirely concealed under the 

 overlapping anterior margin of the body (Fig. 154). The fe- 

 males of this family do not become distended as do those of the 

 Ixodidae, but take more moderate though more frequent meals. 

 They are chiefly inhabitants of warm countries. Both nymphs 

 and adults feed at night, usually dropping off their hosts im- 

 mediately after a meal, and thus seldom being carried from the 



