190 BRITISH TYROGLYPHID^E. 



Its apparent length therefore depends somewhat on 

 position. The extent to which the mandibles, or one 

 of them, are or is protruded also affects the apparent 

 form of the rostrum. 



The colour of many, indeed most, of the species 

 depends greatly upon what food they have been eating ; 

 the colour seems to be not merely in the alimentary 

 canal but diffused almost all over the body. 



Nymphs of course vary greatly in size according to 

 age; it is scarcely possible to guess the age from the 

 external appearance; particularly as the females are 

 usually larger than the males, and thus a young 

 female nymph may be as large as an older male nymph ; 

 but there are not any external genital organs to judge 

 sex from. 



In the outline drawings of parts of the creatures 

 some of the smaller hairs are sometimes omitted when 

 they render articulations, etc., obscure. 



Where a species varies greatly in size, in different 

 individuals, the known amount of the variation in 

 length is usually given in the first line of the measure- 

 ments; in the second line a typical length is given; this 

 means that the typical length is a medium, and one of 

 the commonest lengths of English specimens ; and that 

 the other measurements, viz., those of breadth, length 

 of legs, etc., are proportionate to the typical length, and 

 would correspond to a specimen of that length ; this is 

 useful, for although the actual size of the creature is 

 often not of much value in identifying species, yet the 

 proportions of the creature vary but little and are very 

 useful ; of course, when creatures are distended by food, 

 eggs, etc., a certain amount of distortion takes place. 



TYROGLYPHID-E. 



Atracheate Acarina ; usually with soft cuticle, which 

 is smooth or granular; never raised into parallel un- 

 dulating folds ; rarely with smooth chitinised cuticle. 



