MITES AND TICKS 195 



of undesirable prey. The predators investigated included various 

 aquatic insects and fishes, most of which rejected the water mites, 

 but sticklebacks and newts ate them with alacrity. A survey of the 

 literature regarding the stomach contents of fishes and inverte- 

 brate predators has shown that water mites are seldom eaten in 

 nature (Cloudsley-Thompson, 1947) and more recently it has been 

 shown by experiment that nymphs of Aeschna grandis learn to 

 avoid Hydrachna spp. 



The integument of many Acari is provided with glands which 

 may produce distasteful and poisonous secretions and these are 

 particularly evident among the water mites. In addition, most 

 mites are covered with hairs and setae. Many of these are no doubt 

 sensory, but others serve to protect their owners from attack by 

 predators in the same manner that the quills of a porcupine pro- 

 tect it. The shape and form of these setae are legion. 



In contrast to many of the Acari, some of the ticks do not appear 

 to be distasteful and may even be coloured so that they blend 

 inconspicuously with the body of their host. In this way they may 

 not only escape the attentions of the host itself, but also of tick- 

 birds and other potential predators. 



The eggs of the tick Dermacentor andersoni contain a toxic prin- 

 ciple and, when inoculated in quantity into experimental animals, 

 may cause death. It is not impossible that this may have been 

 evolved as a deterrent to possible predators. No doubt further 

 work would produce evidence to show that the bright colours of so 

 many mite eggs are a form of warning advertisement, like the 

 colours of the immature and adult stages. 



Reproduction and life cycle 



Nearly all mites lay eggs, although a few of the Oribatei are 

 ovoviviparous. Occasionally the mother may die at a time when her 

 abdomen contains a few ripe eggs, and these are able to complete 

 their development internally so that fully-formed larvae emerge 

 from the dead body of their parent. The young undergo meta- 

 morphosis varying in completeness in the different groups. Al- 

 together five or six stages can be recognised, but they are seldom, 

 if ever, all exhibited in the development of a single species. The 

 life history normally consists of the egg which, is some cases is 



