THE LIFE-HISTORY OF HYDROBIUS FUSCIPES, L. 339 



DESCRIPTION OF PLATES. 



Plate I. 



Fig. 1. Right mandible of imago, showing the grinding region (a). Partial view of the lower side 

 and of the " inner " face. 



Fig. 2. Left mandible, showing concavity (b) into which the grinding portion of the right mandible 

 fits. Partial view of the lower side and of the " inner " face. 



Fig. 3. First maxilla of left side of imago showing the series of "brushes" which assist in the feeding 

 process, (a) cardo, (b) stipes, (c) palpiger, (d) galea, (e) lacinia, (/) the transparent membrane of the stipes. 

 m., muscle attachments. 



Fig. 4. The labium from below, m., mentum ; sm., submentum. 



Fig. 5. Dorsal view of a larva recently hatched ( x 42) ; dotted portions are chitinised. 



Fig. 6 represents the maxillae and labium of larva as seen in a moulted skin by removing the frontal 

 sclerite with the antennas and mandibles. Note the upper face of the submentum (s.) with its outwardly 

 pointing teeth, m., first maxilla; l, labium. Maxilla composed of (1) cardo; (2) stipes bearing (3) 

 undifferentiated galea and lacinia, and (4) palpus. 



Fig. 7. The frontal sclerite of head of larva with antennas and mandibles as removed from fig. 7. lr., 

 labrum. 



Figs. 8 and 9 represent dorsal and lateral views of the posterior abdominal segment of the larva, (a) 

 anus ; (b) dorsal sclerite; tr., lateral tracheal trunks; v., their vestibules (these are seen through the dorsal 

 sclerite and open below its posterior ridge) ; (c) the lateral appendages or so-called " cerci." 



Plate II. 



Fig. 10. Dorsal view of penis. The abdominal segments are only partially extended. In numbering the 

 abdominal segments I have followed Peytoureau. 7' and 8', the tergites of these segments ; 7, 8, and 9, 

 the sternites ; a., accessory lobe of penis. ( x 60.) 



Fig. 11. Ventral view of penis, the connective tissue regions between segments 7 and 8 and 8 and 9 

 being fully extended. Lettering as in fig. 10. ( x 60.) 



Fig. 12. Ventral view of 9 genital armature. 7 and 8, the abdominal segments, (a) Base of one 

 of the spinnerets (b) ; [for b' see text] ; (c) the ventral grooved plate ; (x) one of the two pivots upon 

 which the spinnerets swing ; (y) the chitinised lateral portion of segment 8. ( x 60.) 



Fig. 13. Lateral view of the $ genital armature, both vagina and rectum being prolapsed. Lettering 

 as in fig. 12. ( x 60.) 



Fig. 14. Ventral view of ? genital armature in a state of rest showing the spinnerets retracted. 



Plate III. 



Figs. 15-31. The development of the egg. 



Fig. 15, 2-3 days stage, showing primitive streak (p.s.) and commencement of the amniotic cavity 

 (am.c.) at posterior end of egg. j;er., peri vitelline space. 



Figs. 16 and 17, 3-4 days stage, a./., amniotic folds. The amnion (am.) has grown over the jtosterior 

 part of the primitive streak towards the head, the lateral folds of which (hi.) are to be seen backed by the 

 lateral folds of the amnion, bl., blastopore. 



Fig. 18. 5 days stage. (There is a stage wanting between this and that of the last figure owing to 

 the three earlier drawings having been taken from an egg at a different time of year showing a different rate 

 of development.) The embryo is seen bent backwards round the yolk, the appendages being visible as buds. 



Figs. 19 and 20. 6 and 7 days stages showing the gradual shortening of the embryo within the egg 

 and the development of the appendages. 



Fig. 21. 8 days stage. The embryo showing the ruptured amnion twisted on the back to form the 

 dorsal organ (d.o.). 



