ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 431 



will disappear wLcn metamorphosis takes place, an ectoderm, and an 

 cudoderm. The ectoderm is divisible into an optogenic and a non- 

 optogenic region ; the former contains large fusiform cells which are 

 regularly arranged, and are destined to form the elementary eye — 

 these are the optogenic cells ; with each of them a nerve-cord is con- 

 nected ; the post- retinal fibres pierce the basal or internal limiting 

 membrane of the optogenic region, and passing backwards form a 

 thick layer ; morphologically they are comparable to the mesoderm of 

 the ordinary discs ; they then become arranged in a cylindrical bundle 

 (nerve-trunk) which passes to the optic ganglion. The optic ganglion 

 lies between the nerve-branch and the optic nerve, and is formed of 

 the same essential parts as in the imago, but these instead of being 

 separated from one another, are so closely packed as to give the ganglion 

 the appearance of a globular mass ; the medullary masses and chias- 

 mata are placed in the centre, while the ganglion masses are peripheral 

 in position. The optic nerve arises from the two constituent capsules 

 of the internal mass, is very short, and is entirely hidden ; it is formed 

 of two perfectly distinct bundles, one, superior, which passes to the 

 anterior region of the brain ; and one, inferior, which goes to the lateral 

 parts. The optic ganglion is invested in a double neurilemma, which 

 is continued on from the cerebral investment ; between it and the ner- 

 vous tissues there are to be found parts epithelial in character, which 

 probably play an important part at the moment of metamorphosis ; 

 the perilaminar portion is in the form of an epithelial band which 

 occupies a deep groove on the surface of the ganglion, while the intra- 

 ganglionic portion is placed in the posterior part of the ganglion, and 

 is superficial for a small portion only of its extent. 



Anatomy of Psyllidae* — Dr. E. Witlaczil describes the external 

 form of these insects, which calls to mind that of the Cicadellidse. 

 The head is well developed, the thorax, which contains the muscula- 

 ture for flying and springing, is strong, but the abdomen is compara- 

 tively weak and somewhat elongated ; in the last there is the typical 

 number of ten abdominal segments, one of which, as in many other 

 insects, appears to strengthen the thorax ; in the penultimate (male), 

 or ante-penultimate (female), there are appendages which correspond 

 in number and form to those of many other insects. There are greater 

 differences in the form of the larvae than of the imagines, and this is 

 probably due to their parasitic mode of life ; as a rule, the body is 

 compressed dorsoventrally, broad, pretty thick, and well rounded off. 

 There appear to be generally four larval stages which are separated 

 from one another by ecdyses. 



The skin, the fat-body, and the musculature agree generally with 

 those of the Aphides ; wax-glands of altogether similar function to 

 those of the gall-dwelling Aphides are to be found around the anus of 

 the larvae, and of the adult imago, and as in them, they owe their 

 origin to cells of the hypodermis. In a large number of larvse 

 jjeculiar hair-like structures of various forms, which the author calls 

 wax-hairs, are to be found on projections of the integument ; these 



* Zcitachr. f. Wiss. Zuol., xlii. (1885) pp. 568-638 (3 pis.). 



