AKACHNIDA. 363 



a light of great brilliancy ; and in Italy, during the summer nights, the 

 groves, illuminated by their incessant scintillations, exhibit a scene 

 equally strange and beautiful. The females of Lampyris noctiluca have 

 two large yellowish-white luminous plates upon the ventral surface of 

 the sixth and seventh abdominal rings, and, besides these, two minute 

 organs of a similar description on the eighth or caudal segment. The 

 latter only (and these of a smaller size and greyish transparent hue) are 

 present in the males. 



(934.) All the luminous organs, both ventral and lateral, present 

 essentially the same structure, consisting of an investing membrane 

 enclosing a parenchyma composed of tracheae and nerves surrounding- 

 groups of cells so densely filled with white, spherical, minute granules, 

 having an oily aspect when viewed by transmitted light, that no other 

 constituent can be seen in them ; and from experiments, as well as the 

 anatomical facts, Kolliker concludes that the luminous organs are a 

 nervous apparatus, whose nearest analogues are to be sought for in the 

 electrical organs of certain fishes*. 



CHAPTER XIII. 



AKACHNIDAt. 



(935.) THE Arachnidans, long confounded with INSECTS, and de- 

 scribed as such even by recent entomologists, are distinguished by 

 characters of so much importance from the animals described in the 

 last chapter, that the necessity of considering them as a distinct class 

 is now no longer a matter of speculation. In INSECTS, the external 

 skeleton presents three principal divisions the head, the thorax, and 

 the abdomen : but in the Spider tribes, the bloodthirsty destroyers of 

 the insect-world, the separation of the head from the thorax, which, 

 by increasing the flexibility, necessarily diminishes the strength of the 

 skeleton, is no longer admissible ; and, the process of concentration 

 being carried a step further, the head and thorax coalesce, leaving onlv 

 two divisions of the body recognizable externally, viz. the cephalo- 

 thorax and the abdomen. Insects, in their mature forms, were found 

 to be invariably furnished with only six legs, but in the adult Arach- 

 nidans eight of these limbs are developed. These characters in them- 

 selves would be sufficient to discriminate between the two orders ; but 

 when to these we add that in the Arachnidans the eyes are invariably 

 smooth, that the antennae of Insects are represented by organs of a 



* Verhandl. d. Wiirzb. Phys. Med. Ges. viii. 1857. 

 t 'Apd%vr], a spider. 



