332 THE MITES 



the pharynx. The stomach has pouches opening from it which 

 act as food reservoirs (Fig. 149), so that one meal may last for 

 a long time. The intestine is usually short and the excretory 

 organs, malpighian tubules, open into it not far from the anus. 

 The reproductive organs, as in other Arachnida, open on the ventral 

 surface of the abdomen but at different places in different species. 

 The nervous system is largely concentrated into a great mass, 

 the " brain," lying near the anterior end of the body and pierced 

 by the oesophagus. Many mites possess tracheae, similar to 

 those of spiders and insects, for breathing, while others, soft- 

 skinned forms, simply absorb oxygen through the surface of the 

 body. 



Life History. The life histories of mites and ticks are some- 

 what variable, but usually there are four stages in their develop- 

 ment: the egg, the larva, the nymph and the adult (see Fig. 157). 

 The eggs are usually laid under the surface of the soil or in crev- 

 ices, or, in some parasites, under the skin of the host. After a 

 varying period of incubation, which depends on climatic con- 

 ditions, the larva hatches in the form of a six-legged creature, 

 often quite unlike the parent. After a single good feed of blood 

 or plant juices the larva rests, sheds its skin and appears with 

 an additional pair of legs and a body form more closely resem- 

 bling that of the parent but without developed sexual organs. 

 The nymph thus produced feeds again, sheds its skin from one 

 to three times and finally, after another period of rest during 

 which its body is remodeled for the second time, moults again 

 and comes forth as a fully adult male or female, ready for the 

 reproduction of another generation. There are all sorts of modi- 

 fications of this order of development, due to the slurring over of 

 one phase or another. One of the most aberrant species is the 

 louse-mite, Pediculoides. In this form the eggs develop within 

 the parent's body and the adult males and females issue forth 

 from the brood chamber improvised for them out of the abdomen 

 of the mother (Fig. 139). 



The popular opinion that all mites are parasitic is, as remarked 

 before, far from being true. Over half of the known species are 

 not parasitic at any stage in their life history, while many others 

 are parasites only during part of their life cycle. 



Parasitism. The mites are an interesting group for the study 

 of the origin of parasitic habits since, as Ewing has shown, para- 



