ilOUJ^TIXG AND PRESRRVATION OF DIPTBUA, 



33 



prefer not to guiii the specimen into the pith, so that it can be 

 removed for closer microscopic examination when necessary. In 

 the case of the excessively minute forms they should be pierced 

 from below the body, tlie point only o£ the very smallest pin 

 bein<^ inserted so as only just to emerge from the dorsum, the 

 head^'end of this pin (there being, as said before, no real enlarg- 

 ment as a " head") being stuck in tlie pith, but in tins case it is 

 most advisable to gum the pin in the pith, it being much more 

 likely to fall out when inserted, so to speak, upside down. 



For the closer examination of the anatomical structure of very 

 small Diptera, as Psychodid.tj and all those of still lesser size, it 

 is convenient to mount some of the specimens on microscopic 

 slides, preferably in a more or less dissected state ; but this 

 method has at least one disadvantage, that it frequently destroys 

 all the exterior characters, such as pubescence, spines, bristles, 

 all of which are valuable specific points of difference. 80 tliat 

 unless the species depends solely upon some anatomical structure 

 quite invisible to the naked eye, it should be identified whenever 

 possible before mounting it on a slide, as it is otherwise often 

 impossible'to do so, many of its external characters being after- 

 wards unavailable. The venation of some of the very pubescent 

 PsYCHODiD.E is absolutely invisible until the wings are denuded of 

 the very thick hairs with which they are covered, augmented in 

 many cases by the addition of semi-opaque imbricating scales. 

 It is very dithcult to determine a single specimen of such a species 

 unless oiie wing be broken off and mounted for the microscope,, 

 the other one remaining intact on the dried specimen. 



A new process has somewhat recently come to my notice; this, 

 is the imbedding of the specimen bodily in an alcoholic solution of 

 collodion, with a backing of a mixture of oxide of zinc and Canada- 

 balsam, the collodion protected above by a " cover slip " of glass^ 

 and the whole mounted on a microscopic slide. The head, wmgs 

 and legs are arranged so as to be easily examined and there is no 

 danger of the specimen meeting with any damage. 



Tins process, although it has the advantage of presenting a 

 perfectly stereoscopic view when seen under the microscope, has 

 the serious disadvantage of only one surface, dorsal or ventral,, 

 beino- visible in any individual specimen ; so that unless the 

 identification of each specimen be done by the collector (who in 

 very many cases is himself not a specialist) before mounting, 

 considerable doubt is liable to attach to the specimens from the 

 inability of tlie actual specialist to examine both sides of the 

 same individual. In the cases of very closely allied species known 

 to occur in the same neighbourhood the positive identification of 

 a large proportion might be impossible. 



The onlv way of retaining specimens permanently in good 

 dition* after being properly mounted is by their inclusion in 



cone 



* i collection of Diptera, if the specimens be thoroughly dried at the 

 start' and preserved in a cabinet as described, in a temperate climate, should 

 remain in good condition for not less than a hundred years. 



B 



