ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 401 



differ from ordinary skin- glands in having a larger nucleus, a denser cyto- 

 plasm, with a smaller ellipsoid body and a less distinct radial striation. 



Suckers of Dytiscus.*— Hans Blunck gives a detailed description 

 of these familiar instruments. There is a mechanical attachment which 

 works after death, and there is a secretion on the sole of the anterior and 

 middle tarsals. This helps adhesion indirectly, like grease in an air- 

 pump, and directly by increasing the adhesion-force. The question of 

 the measurement of this adhesion- force is discussed. It seems that the 

 only certain use of the suckers, which are confined to the males, is to 

 facilitate adhesion durinsr sexual union. 



o 



Eyes of Dytiscus. t — Karl Giinther has studied the minute structure 

 of the eyes in the larva and imago of Dytiscus marginalis. He deals 

 with the ocelli or stemmata, the eye-spot, the condition of the stemmata 

 during metamorphosis, and the structure and development of the com- 

 pound eye. The stemmata show two kinds of receptor-elements — hori- 

 zontal rods with small visual cells and vertical rods with large visual 

 cells. The rods are in general to be regarded as hollow structures which 

 embrace the distal ends of the visual cells. Besides the six stemmata 

 there is an eye-spot, with a lens-like body, distinct rods, and a connexion 

 with the optic ganglion. The stemmata resist dissolution during meta- 

 morphosis, persist in the imago, though without lens and vitreous body. 

 In the development of the retinula the central cell in each group of eight 

 migrates towards the base ; one of the seven radials is dislodged from 

 the retinula ; the basal and the six radials form the rhabdom. The 

 rhabdom is surrounded by a clear mantle — the insertion surface of very 

 fine nerve-endings. There is iris pigment and retina pigment formed 

 from pigment cells and retina cells respectively. 



Sensory Nervous System of Cockchafer Larvae. J — A. Zawarzin 

 devotes the third of his histological studies on insects to this subject. 

 Two kinds of cells are concerned. One set form a continuous subcu- 

 taneous plexus, and are connected w'ith the central nervous system. 

 The others are ordinary receptor cells, which innervate the sensory 

 organs. In some insects, such as earwigs and ^schna larv^, the 

 sensory nervous system consists almost exclusively of ordinary receptor 

 cells ; in others, such as caterpillars and cockchafer larvfe, the subcu- 

 taneous plexus of cells with branched peripheral processes is more pre- 

 dominant. 



Development of Aphides. § — Jan Hirschler gives an account of the 

 segmentation, the formation of the blastoderm, and the development of 

 the gut. The secondary endoderm is poorly developed in the summer 

 eggs of Aphides, being restricted to a region at the posterior end and 

 another behind the head-lobes. There is no endodermal band along the 

 whole blastoderm, as in many insects. The yolk-cells take no part in 

 forming the embryo. The gut owes its origin to the derivatives of two 



* Zeitsebr. wiss. Zool., c. (1912) pp. 459-92 (11 figs.). 



+ Zeitschr. wiss. Zool., c. (1912) pp. 60-115 (36 figs'). 



X Zeitschr. wiss. Zool., c. (1912) pp. 447-58 (1 pi. and 1 fig.). 



^ Zeitschr. wiss. Zool., c. (1912) pp. 393-446 (2 pis. and 7 figs.). 



