464 ARACHNIDA—ACARINA CHAP. 
they are analogous to the winged individuals which occur in the 
parthenogenetic generations of the Aphidae. The ordinary 
Tyroglyphus is soft-bodied, and requires a moist environment, and 
exposure to the sun or prolonged passage through the air would 
be fatal to it. The hypopial form is much more independent of 
external conditions, and its habit is to attach itself by its suckers 
to various insects, and by this means to seek a new locality, when 
it resumes the ordinary nymph-form and proceeds with its 
development. 
Classification.—There is no generally accepted classification of 
the Acarina, though several eminent Arachnologists have attempted 
of late years to reduce the group to order. Widely different 
views are held concerning the affinities of certain groups, and 
there is no agreement as to the value to be accorded to the 
characters which all recognise. Thus Canestrini' allows thirty- 
four families, while according to Trouessart * there are only ten. 
Trouessart’s scheme of classification 1s in the, main followed 
in the present chapter. 
Sub-Order 1. Vermiformia. 
This Sub-order includes the lowest and most aberrant forms 
of the Mites. They are entirely parasitic, and of very small size. 
The abdomen is much elongated, and is transversely striated. 
There are two families, Eriophyidae* (Phytoptidae) and Demo- 
dicidae. 
Fam. 1. Eriophyidae (Phytoptidae).—These are the so-called 
Gall-mites. The curious excrescences and abnormal growths which 
occur on the leaves and buds of plants are familiar to every one. 
Various creatures are responsible for these deformities, many being 
the work of insects, especially the Cynipidae among the Hymen- 
optera, and the Cecidomyiidae among the Diptera. Others, again, 
are due to Eriophyid Mites. 
Though the galls originated by Mites are often outwardly 
extremely similar to those of insect origin, they can be at once 
distinguished on close examination. Mite-galls contain a single 
chamber, communicating with the exterior by a pore, usually 
Atti Ist. Veneto, u1., 1891, p. 699. 
cev. Sci. Nat. Ouest, ii., 1892, p. 20. 
° Hriophyes, v. Siebold, Jahresber. Schles. Ges. xxviii., 1850, p. 89; Phytoptus, 
Dujardin, Ann. Sct. Nat. (3), xv., 1851, p. 166. 
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