02 GNATS OR MOSQUITOES — CHAPTER VI 



surface of the similarly lielcl together hind tarsi, by which 

 they are guided to the surface of the water on which they 

 are to be launched. 



If we dissect the abdomen by teazing with needles or 

 by sectionising, we find that immediately beneath the 

 chitinous exoskeleton there is a soft, cellular layer, the 

 true dermis ; and beneath this again, a very scanty and 

 interrupted layer of longitudinal muscular fibres, which 

 not only serve as flexors, extensors and abductors, but 

 can also, when simultaneously contracted, shorten longitu- 

 dinally and therefore transversely widen the entire region, 

 an action which cannot fail to have an effect in keeping 

 in motion the air contained within the tracheal system. 

 For although there is no continuous, rhythmic action of 

 the body comparable with the respiratory movements of the 

 higher air-breathing animals, movements of this kind are 

 sufficiently frequent and habitual to exercise a powerful 

 action in preventing stagnation of the air contained in the 

 respiratory tubes. These longitudinal muscles are arranged 

 in sets, each set corresponding to an intersegmental mem- 

 brane and serving to connect two adjacent segments. There 

 are also a few transverse fibres, arranged mainly in two 

 lateral groups, placed near the middle of the segments, but 

 they are less easily made out. 



The best way to exhibit them is to carefully remove all 

 the scales from the abdomen, a preparatory step which 

 should be taken as a routine matter in all dissections, by 

 gentle brushing with a camel hair pencil. Dip the specimen 

 for a moment in spirit, and then by a stroke of a pair of 

 fine scissors split the abdomen into lateral halves. Ex- 

 amined in salt solution, such a preparation, after cleaning 

 away the viscera by means of the needles, shows well the 

 stigmata and parietal trachea;, and subsequently, after 

 appropriate staining in borax-carmine, will give a good view 

 of tliese parietal muscles. The precaution of dipping the 

 specimen into spirit for an instant, before placing in salt 

 solution, is a necessary preliminary to all dissection of these 

 insects, as otherwise, however carefully one may have 

 brushed, one is sure to be embarrassed by air bubbles, 

 obstinately entangled in the remaining tomentum. 



