2S8 ZOOLOGY FOR MEDICAL STUDENTS chap. 



provided with recurved hooks^ and these organs when thrust into the 

 skin of the host give a firm attachment. If the tick is forcibly pulled 

 off, these organs are left behind in the skin and may continue to cause 

 irritation for weeks. A tick should therefore be induced to detach itself 

 by applying to its body a small drop of oil which will get into its stigmata 

 and usually cause it to release its hold. 



The majority of ticks are grouped together as the Ixodidae : they 

 are recognizable by a stiff chitinous plate which covers in the male the 

 greater part of the dorsal surface, and in the female the fore part, all the 

 hinder portion being enclosed in tough leathery skin. Ixodes is the 

 common sheep-tick .^ Boophilus and Rhipicephalus are of practical im- 

 portance as transmitters of Piroplasma or Babesia (p. 64). 



The Argasidae are without the stiff chitinous plate on the dorsal 

 surface and the body bulges forwards so as to conceal the head region 

 in a view from the dorsal side. Ornithoiorus, common about native 

 huts and camping spots in tropical Africa, is the transmitter of African 

 Relapsing Fever (p. 77). 



The Tarsonemidae are very minute mites which cause injury to 

 plants. A species of Tarsonemus has been found recently infesting the 

 tracheal tubes of Bees suffering from " Isle of Wight " disease, and is 

 believed by its discoverers to be the cause of the disease. 



The Trombidiidae include the red Harvest mites the larval stage of 

 which (" Harvest bug " ; Bete rouge ; Bicho Colorado) is apt to get on 

 to the skin of human beings in grassy country and causes intolerable 

 itching by its bite. 



V. The Crustacea include a vast variety of arthropods adapted to 

 an aquatic existence. In correlation with this the function of breathing 

 is carried out by gills — thin-walled projections of the body-surface spring- 

 ing as a rule from the appendages. And very commonly some of the 

 appendages are, during at least part of the life-history, modified for 

 swimming. The cuticle is usually stiffened by the deposit, within its 

 substance, of salts of lime. The life-history commonly involves great 

 changes in form between the larval and the adult phase, but these changes 

 are brought about in successive steps without that re-organization of 

 practically the whole body that occurs in the metamorphosis of insects. 



A conspicuous feature which usually enables a crustacean to be recog- 

 nized at a glance is afforded by the presence of two pairs of antennae. 



The Crustacea are commonly classified in two main groups, the Mala- 



1 The latter name is sometimes erroneously applied to the "sheep-ked" 

 (p. 254). 



