616 CLASS IV. INSECTA. 



Lepidoptera they are loosely linked together by a structure 

 termed the frenulum and retinaculum, whilst in the Hymenoptera 

 they are closely knit together by a row of hooks on the anterior 

 edge of the hind- wing which play over a ridge on the posterior 

 rim of the fore-wing. 



The rapid vibration of the wings of insects, which in the house- 

 fly attains the number of 335 beats a second and in the bee 440 

 beats per second, makes a perceptible noise. The humming 

 or buzzing which is so characteristic a feature of the Diptera and 

 Hymenoptera seems to be due to a fundamental note caused 

 by the vibration of the wing through the air and to certain 

 overtones. A mechanism which would produce such overtones 

 has recently been described * at the base of the wings in certain 

 Diptera. It consists of a toothed bar which works against a 

 ridged blade and it is thought that as the wing is raised and 

 lowered these two structures grate against one another (Fig. 368). 



No one now holds the view that the wings are homologous with 

 the true appendages, ^ though their real nature is still a matter 

 of dispute. The view was formerly put forward that they 

 are modifications of the plate-like tracheal gills such as those 

 borne on very varying parts of the body by the larval Sialidae, 

 Perlidae, Odonata, etc., etc. They certainly resemble the 

 tracheal gills in being supplied with tracheae and with blood, 

 but the details of the tracheal system differ markedly in the 

 two structures, and the view is now abandoned. It is more- 

 over evident that whereas the gills arise at very varying parts 

 of the body and appear to be temporary adaptations to an 

 aquatic life, the wings arise always from the meso- and meta- 

 nota. Thus Fritz Miiller's J view that wings are modifications 

 of lateral outgrowths of the dorsal surface of these segments 

 and not of any larval organ has much to commend it. 



The third division of the body or abdomen is not so inelastic 

 and compacted together as are the head and thorax. It is 

 capable of considerable extension, a necessary consequence of its 

 containing the larger part of the alimentary canal and the ovary, 



* On a possible Stridulating Organ in the Mosquito (Anopheles maculi 

 pennis Meig.) by A. E. Shipley and E. Wilsor, Tr. Roy. Soc. Edin., xl. 1902, 

 p. 367. 



f A typical segment bears a pair of appendages usually directed ventrally, 

 such appendages form the gnathites and the legs, and are referred to 

 here as true appendages. 



f Jenaische Zeitschr. Naturwiss., ix, 1875, p. 241. 



