188 BRITISH TYROGLYPHIM. 



life-history of every individual, but only occurring in 

 the development of some specimens without any known 

 rule. 



The drawings of whole creatures in this book have 

 in every instance been made from living specimens; 

 this is almost a necessity because the cuticle in the 

 Tyroglyphidse is usually only slightly chitinised, and 

 therefore the shape is apt to alter considerably after 

 death ; I have, however, always had a second specimen, 

 either dead and unmounted or mounted without presure, 

 ready for constant reference as to detail ; this is most 

 conveniently kept under a second microscope. All 

 whole figures and sections and the larger dissections 

 have been drawn by dropping a piece of glass ruled 

 into squares into the eye-piece and drawing on paper 

 also ruled into squares (engineer's sectional paper), 

 drawing square for square. This, in my opinion, 

 gives a more correct result than the camera, which 

 necessarily always produces a certain amount of dis- 

 tortion ; even the squared-glass system, which I use 

 and believe to be the best, produces a slight amount of 

 distortion ; because the ruled glass being between the 

 field-lens and the eye-lens of the eye-piece the correc- 

 tion of the error of the eye-lens by the opposite error 

 of the field-lens is not obtained ; but the distortion is 

 far less than that of the camera. 



The smaller dissections, such as legs, mandibles, etc., 

 have been drawn by the aid of a Zeiss two-prism 

 camera. With these very minute objects the amount 

 of distortion is a negligible quantity. 



The scale of each figure, except those of larvae and 

 nymphs, is given in the explanation of the plates ; it 

 would be useless to give it in drawings of the immature 

 creatures because it would depend on the exact age of 

 the specimen drawn from. The fully-grown nymph is 

 generally slightly smaller than the imago. The hypo- 

 pial nymph is an exception from this rule ; the 

 Hypopus does not grow 5 and therefore its size is 

 constant. 



