Characteristics of Insecta. cxi 



placed posteriorly to it may be ' represented by a single continuous 

 mass giving" off nerves laterally and posteriorly ; or they may take 

 the shape of a chain of ganglia, which are never more than eleven, 

 three being thoracic and eight abdominal, though by fusion or 

 abortion they ordinarily fall below this number. The sympathetic 

 system, both in its stomato-gastric division, and in that part of 

 it which is in connection with the ventral ganglia and supplies the 

 tracheae, attains in insects a large development. The eyes are 

 always confined to the head; in a few instances, of which the 

 Chloeon dimidiatum, one of the Orthoptera Amphihiotica, already 

 mentioned, is one, as also in certain Dipterous, Strepsipterous, and 

 Hemipterous genera, the eyes are elevated upon peduncles or pillars, 

 which however are never movably articulated to the head. In 

 most orders of insects, but most frequently amongst Hymenoptera, 

 Diptera, and Orthoptera, the so-called ' simple eyes,' s. ' stemmata,' 

 s. ' ocelli,' coexist with the larger multifacetted eyes. 



Tlie males and females are ordinarQy very different in insects ; 

 the males, except in the cornuted species, being slighter in make, 

 swifter, furnished with larger eyes and antennae, and more brightly 

 coloured. The difference may be so great, especially when the 

 females are apterous and the males winged, as to amount to a kind 

 of Dimorphism. 



The generative organs of Insects are very varied in the details of 

 their arrangement. The reproductive glands are always double 

 and symmetrical, but the efferent ducts always fuse into a common 

 duct before opening. This they do posteriorly to the eighth abdo- 

 minal segment ; which is homologous with the third caudal segments 

 of Scorpionidae and Crustacea, and therefore posterior by eight seg- 

 ments to the genital segment of the Arachnida just named, and to 

 that of Limidus ; and by three to the hinder of the two genital 

 segments of the Decapodous Crustacea. The female generative 

 organs of insects have often a large number of accessory appendages ; 

 the most constant of these is the ' receptaculum seminis ;' but there 

 may be present also an ' accessory gland' appended to the ' recepta- 

 culum seminis;' secondly, a 'bursa copulatrix;' and thirdly, a num- 

 ber of ' colleterial ' glands which secrete a glutinous material lor 

 fixing the ova to various external objects. The male accessory organs 

 are of two kinds, one of which is considered as analogous to the pro- 

 static organ, and the other to the vesieulae seminales of Mammals. 



