464 ARACHNIDA ACARINA CHAP. 



they are analogous to the winged individuals which occur in the 

 parthenogenetic generations of the Aphidae. The ordinary 

 TyroglypUus 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 l allows thirty- 

 four families, while according to Trouessart 2 there are only ten. 



Trouessart's scheme of classification is 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 3 (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 



1 Atti 1st. Veneto, ii., 1891, p. 699. 



2 Rev. Sci. Nat. Quest, ii., 1892, p. 20. 



3 Eriophyes, v. Siebold, Jahresber. Schles. Ges. xxviii., 1850, p. 89 ; Phytoptus, 

 Dujardin, Ann. Sci. Nat. (3), xv., 1851, p. 166. 



