VI ARACHNIDA 257 



sticky secretion which runs together into tiny droplets studded along the 

 course of the thread. 



The Phalangidea include a great variety of creatures, some of them 

 exceedingly common and usually mistaken for long-legged spiders. They 

 are easily distinguished from the true spiders by there being no con- 

 stricted " waist " between the prosoma and the opisthosoma. 



The AcARiNA, including the Mites and Ticks, is the subdivision of the 

 Arachnida which is of the greatest practical importance — many of them 

 leading a parasitic existence on other animals or plants. As in the 

 preceding group there is no marked " waist " ; the mouth-parts are 

 usually adapted for sucking. Tracheal tubes are present in the larger 

 Acarines but in many of the small-sized mites, in which the cuticle is 

 very thin and the ratio of surface to volume relatively great, the respira- 

 tory exchange takes place through the general surface of the body and 

 there are no special breathing organs. 



A few of the mites have an elongated worm-like shape. Some of 

 these live on the leaves of plants, producing galls or causing distortion 

 of the leaves, as is the case with Eriophyes or Phytoptus, common on the 

 leaves of the black currant : others are parasitic on animals as e.g. 

 Demodex, the " Blackhead," a common parasite in the hair-follicles of 

 the face of man. 



The Sarcoptidae include the Itch-mites of the genus Sarcoptes 

 which inhabit the skin of mammals, causing the diseases known as 

 Mange and Scabies or Itch. Where large numbers of human beings are 

 crowded together, as they are liable to be on active military service, 

 Scabies is liable to spread and affect a large proportion. 



The adult mite burrows along in the substance of the skin. The 

 female is fertilized in the burrow, and in the course of its life, which lasts 

 at least three weeks, it deposits 20-50 eggs. After 2^-t,\ days the egg 

 gives rise to a six-legged larva, and this in 1^-3 days to the eight -legged 

 pupal or " nymph " stage. After 1-2 days the sexual stage is reached. 



There are various other genera allied to Sarcoptes. Tyroglyphus is 

 the cheese-mite, while Glyciphagus sometimes becomes a serious pest 

 through its infesting in myriads household furniture. The latter mite 

 is exceedingly difficult to get rid of, for even fumigation or " gassing " 

 with the most poisonous gases is apt to leave in inaccessible crannies a 

 few survivors which are sufficient to start a fresh invasion. 



The Ticks (Argasidae and Ixodidae) are much larger than the other 

 mites : they may reach an inch in length when distended with blood. 

 When undistended the body is flattened. The mouth-parts are adapted 

 for sucking: an unpaired " hypostoma " and a pair of chelicerae are 



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