Various species of Tyroglyphidæ ([fig. 150f]) may abound on dried fruits and other products and attacking persons handling them, may cause a severe dermatitis, comparable to that described above for Pediculoides ventricosus. Many instances of their occurrence as such temporary ectoparasites are on record. Thus, workers who handle vanilla pods are subject to a severe dermatitis, known as vanillism, which is due to the attacks of Tyroglyphus siro, or a closely related species. The so-called "grocer's itch" is similarly caused by mites infesting various products. Castellani has shown that in Ceylon, workers employed in the copra mills, where dried cocoanut is ground up for export, are much annoyed by mites, which produce the so-called "copra itch." The skin of the hands, arms and legs, and sometimes of the whole body, except the face, is covered by fairly numerous, very pruriginous papules, often covered by small, bloody crusts due to scratching. The condition is readily mistaken for scabies. It is due to the attacks of Tyroglyphus longior castellanii which occur in enormous numbers in some samples of the copra.
Sarcoptidæ
The Sarcoptidæ are minute whitish mites, semi-globular in shape, with a delicate transversely striated cuticula. They lack eyes and tracheæ. The mouth-parts are fused at the base to form a cone which is usually designated as the head. The legs are short and stout, and composed of five segments. The tarsi may or may not possess a claw and may terminate in a pedunculated sucker, or simple long bristle, or both. The presence or absence of these structures and their distribution are much used in classification. The mites live on or under the skin of mammals and birds, where they produce the disease known as scabies, mange, or itch. Several species of the Sarcoptidæ attack man but the most important of these, and the one pre-eminent as the "itch mite" is Sarcoptes scabiei.
The female of Sarcoptes scabiei, of man, is oval and yellowish white; the male more rounded and of a somewhat reddish tinge, and much smaller. The body is marked by transverse striæ which are partly interrupted on the back. There are transverse rows of scales, or pointed spines, and scattered bristles on the dorsum.
The male ([fig. 56]) which is from 200-240µ in length, and 150-200µ in breadth, possesses pedunculated suckers on each pair of legs except the third, which bears, instead, a long bristle. The female ([fig. 56]) 300-450µ in length and 250-350µ in breadth, has the pedunculated suckers on the first and second pairs of legs, only, the third and fourth terminating in bristles.
The mite lives in irregular galleries from a few millimeters to several centimeters in length, which it excavates in the epidermis ([fig. 57]). It works especially where the skin is thin, such as between the fingers, in the bend of the elbows and knees, and in the groin, but it is by no means restricted to these localities. The female, alone, tunnels into the skin; the males remain under the superficial epidermal scales, and seldom are found, as they die soon after mating.