ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 499 



consideration the density and other light-absorbing properties of the 

 medium in which the insects live, the amount of light received from 

 an object must vary approximately as the cube of the distance of the 

 source of light, hence the wide opening of the eye, admitting as much 

 light as possible. The same effect as the perspective which is obtained 

 in air is here produced by the indistinctness of objects, owing to 

 the opacity of the medium, in proi)ortion to their distance from the 

 eye. 



The structural causes for these differences are: — (1) the flatness 

 or slight curvature of the Crustacean cornea, which does not hinder 

 the entrance of any light which falls radially upon it, and (2) the 

 strong convexity of the facets in insects, which causes refraction of 

 the rays to a focus in front of the retina, and consequently a dimi- 

 nution of the light which meets them ; thus most of the hurtful ra} T s 

 — those whose direction is not exactly at right angles to the surface 

 of the cornea — having entered the eye at its side, are again thrown 

 to the side and absorbed by the walls and the pigment of the narrow 

 tube, whose diameter at the apex only allows of the entrance of a 

 small central pencil. With regard to the fate of strongly divergent 

 rays, the refractive properties of the cornea would appear calculated 

 to increase their brightness ; but this is the case only with objects at 

 short distances, and has the advantage of giving distinct and recog- 

 nizable images of objects within this range. 



Nervous System of the Strepsiptera.* — The nervous system of 

 the Strepsiptera has not been subject to any special researches. C. Th. 

 von Siebold f only states that these insects (Xenos vesparum) have one 

 thoracic ganglion ; but he does not say anything about the number of 

 cephalic and abdominal ganglia. E. Brandt's researches have been 

 limited to four females and one male of Stylops melitlm, and one 

 female Xenos vesparum, preserved in spirit, the results of which are as 

 follows : — 



1. The cephalic division of the nervous system consists of the 

 ganglion supra-oesophageum only, the ganglion infra cesophageum being 

 absent. 



2. The thoracic division consists of a large ganglion containing 

 five pairs of nuclei ; it is divided into two parts, an anterior and 

 smaller one, corresponding to the ganglion infra-oesophageum and to the 

 first thoracic ganglion of other insects, and a posterior and larger part, 

 which corresponds to the other thoracic ganglia and to some abdominal 

 ganglia. The interior division supplies nerves to the organs of the 

 mouth (like the ganglion infra-oesophageum) and to the first pair of 

 legs. The posterior and larger division of this ganglion supplies 

 nerves to the second pair of wings, to the thorax, and to different 

 segments of the abdomen. 



3. The abdominal division of the nervous system consists of one 

 abdominal ganglion, situated in the last third of the body. It is 



* Abstract by the author of a memoir in Russian, St. Petersburg, 1878. 

 Ann. and Mag. Nat. Hist., ix. (1882) pp. 456-7. 

 t Lehrb. d. vergl. Anat., i. (1848) p. 582. 



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