CHAPTER XI 



LABORATORY TECHNIQUE 



THE examination of insect tissues is conducted for the most part by 

 ordinary laboratory processes, descriptions of which are to be found in 

 the numerous excellent textbooks on the subject. The tissues present, 

 however, certain peculiarities, and though the principles underlying the 

 methods are the same, some modifications are necessary. The chief 

 difficulties met with are due to the minute size of the objects, the great 

 difference in density between the exo-skeleton and the internal organs, 

 and the intimate manner in which the air-containing tracheae are con- 

 nected with the tissues. The following account deals only with methods 

 which the authors have used themselves, and does not pretend to be an 

 exhaustive one. 



A word is necessary with regard to the dissecting microscope, which 

 is an important part of the laboratory equipment. For ordinary work 

 a simple type, such as can be purchased, complete 

 with hand rests, mirror, and lenses magnifying ten ^^^ope"* ""' 

 and twenty diameters, for two pounds or so, is suffici- 

 ent ; a satisfactory though not very convenient one can be improvised by 

 fitting a pocket lens on an upright stand, and placing underneath it a 

 large slide, one-half of which is blackened with ink, or covered on one 

 side with black paper. For more elaborate dissections a good binocular 

 microscope, with objectives and eye pieces capable of giving a magnifi- 

 cation of at least forty diameters, is essential. Such an instrument finds 

 many uses apart from dissection, its large field and stereoscopic vision 

 rendering it particularly helpful in the performance of many delicate 

 manipulations. If it is fitted with an illuminating apparatus, as can 

 be done at a slight extra cost, it affords quite the most convenient means 

 of examining macroscopic specimens, such as flies, larvae and pupae. 

 Specimens which are not pinned are best mounted for examination by 

 fixing them, at the desired angle, to a piece of plasticine, which is 

 placed on a slide. 



For the examination of the chitinous structures, cleared preparations, 

 in which the soft parts have been dissolved out, should be made. This 



