THE LIFE-HISTORY OF HYDROBIUS FUSCIPES, L. 327 



scope without disturbing her. The cocoon is formed just as easily under these circum- 

 stances as under water, provided that the temperature is not too high or as long as there 

 is sufficient moisture in the surrounding air. I found by placing a spinning female in 

 the direct sunshine that, after a short time, the spinnerets, although continuing to work 

 backwards and forwards, produced no silk. At first the silk spun became thicker than 

 usual and then it became lumpy and stuck to the spinnerets, and I think this was due 

 to the heat drying up the silk fluid on the spinnerets. It must be admitted that if this 

 explanation is correct it is difficult to explain how, when the beetle is upside down and 

 working on the under side of the grass, the silk runs out the length of the spinnerets 

 against gravity ! 



About half way up the basal segment of each spinneret and on its inner face is a 

 transparent region of more or less oval shape. This region is beyond the "horns" of 

 the grooved ventral plate when the spinnerets are extended, but is completely covered 

 by them when these organs are withdrawn. Under high magnification ( x 400) this 

 region shows a structure different from that of the rest of the spinneret, and I at first 

 thought that the ducts of the silk glands might open here, but I cannot detect any 

 pores. I think therefore that the silk glands must open to the exterior somewhere 

 below the fringe of the ventral plate, that the silk flows out on the hairs of this and 

 passes on to the spinnerets from them. 



The posterior end of the abdomen of the female is extremely flexible during the 

 spinning of the cocoon, segment 7 moving very freely upon segment 6, and the 

 spinnerets also waving about with independent movement. 



As a rule, shortly before a female has completed her cocoon, a male takes up his 

 position on her back and commences stroking her labium. Within two minutes of 

 completing her cocoon the female accepts the male and copulation lasts about a minute. 

 The male ceases to stridulate the moment he begins to protrude the penis. 



The Development of the Egg. (Figs. 15-31.) 



The development of the egg of Hydrophilus has been described by several authorities 

 such as Heider, Graber, Kowalevsky, etc., and that of Hydrobius appears to be very 

 similar, but Hydrophilus has been studied chiefly from the histological point of view, 

 and I propose to describe the development of the embyro of Hydrobius as seen from 

 day to day through the shell of the living egg. There are certain points which are not 

 referred to by the above authors which are of some interest. 



The egg of Hydrobius is oval in shape, from l'l to 1*4 mm. in length, and bears at 

 one end a small pedicel, this being the end of the egg at which the head of the embryo 

 develops. The newly laid living egg shows no structure under the microscope beyond a 

 shell membrane and granular contents which completely fill the shell. After twenty-four 

 hours the contents have shrunk so that there is now a broad, perivitelline space, and on 

 the contents there are at this time certain irregularities which suggest that cell-division 



