100 



NATURAL HISTORY OF ARTHROPODS. 



are really distinct. Demodex foUiculorum, which sometimes occurs in the hair-follicles 

 and sebaceous glands of man, especially those around the nose, is a minute worm-like 

 form, with four pairs of legs near the anterior end of the body. In the 

 human subject it is comparatively harmless, but either this form or a 

 closely allied species is sometimes very injurious to hides. Dr. Faxon 

 records a case where numerous cowhides, from Illinois and Wisconsin, 

 were seriously damaged by this parasite ; in some of the samples as many 

 as eight or ten pits, some of which penetrated nearly through the skin, 

 were found within the area of one square inch. Each of these pits was 

 filled with a fatty substance containing multitudes of individuals. 

 Similar injury to the skins of hogs has also been reported. But little 

 is known concerning the development of these forms, some authors 

 thinking them oviparous, while others think that the young are born 

 alive. The larvae, like those of most mites, have but three pairs of 

 limbs. 



The members of the genus Sarcoptes are very minute. They have 

 a round or oval body, very short three-jointed legs, the two first pairs 

 po- ^^■{jT^f- terminating in a sucking disc, while in the male the posterior pairs 

 rwm,foihcie-mite, terminate in the same manner, but in the female these end in a long 



greatly enlarged. , . , „ . 



bristle. Several species have been described inhabitmg various animals 



but the most prominent is the S. scabei, the Itch-mite, which produces this disgusting 



disease in unclean people. The connection between the mite and disease was first 



pointed out by Avenzoar, an Arabian physician, in the twelfth 



century. These forms burrow just beneath the skin, especially 



in such protected parts as those between the fingers, the inside 



of the wrist, etc. The female is much the larger, attaining a 



length of about a sixtieth of an inch. The usual remedy is 



sulphur ointment rubbed into the skin. S. canis produces the 



mange in dogs, while other species are found in horses, cattle, 



sheep, etc. These latter are sometimes referred to a genus 



2)ermatodectes. 



Among the more typical foi-ms are the genera Tyroglyphus 



and Typhlodromus. In these the feet are long, four-jointed, f^^' itoi^-mite, greatly 

 t? and terminate with a sucking 



disc, and the mandibles are scissor-like. The cheese 

 mite, Tyroglyphus siro, is a familiar example, and 

 scarcely less so is the flour mite, T. farina. Another 

 species, T. sacchari, is frequently abundant in soft 

 unrefined sugar, but it is rarely, if ever, found in re- 

 fined sugar, which is apparently too hard for its exist- 

 ence. This form is supposed to cause the grocers' 

 itch. One species is of benefit to man ; T. phylloxere, 

 as its name indicates, feeds on the Phylloxera, so in- 

 jurious to the grapevines. Under certain conditions 

 some of the species of Tyroglyphus assume a differ- 

 A h?rd, brown chitinous covering develops 

 within the skin, and then the latter cracks and the 

 new form (originally described as Hypopus) emerges. 



Fig. 134. — Sarcoptes 



Fig. 135. —/xorfcsaJfiipiciiM, white-spotted ent form 

 tick of moose, a. Moutli-parts. b. Six- 

 footed young, rf. Foot with suclcing 

 disc. e. Adult, natural size, gorged 

 with blood. 



