OBSERVATIONS ON COLEOPTERA. 

 CYCLE OF PERIODS IN ONE LARVAL STAGE. 



177 



The Exui'ial Glands. The existence of an exuvial fluid in in- 

 sects was first clearly demonstrated by Newport, but its origin 

 remained obscure and was attributed to diverse sources. It was 

 first shown to be in part at least, due to glandular activity by 

 Gonin in Picris brassicte. Gonin found, especially upon the 

 pronotum, a considerable number of large unicellar glands which 

 were in his preparations definitely seen to be extruding their con- 

 tents between the old and new cuticula to form a part at least of 

 the exuvial fluid. Similar unicellular glands are found in other 

 insects, especially at pupation, in free living forms such as most 

 of the larvae of butterflies and moths and of many leaf feed- 

 ing beetle larvae. In L. dccimlincata there are found from 50 to 

 22$ of these glands upon the pronotum in the last larval stage, 

 and their sole function seems to be to develop an enormous 

 amount of the exuvial fluid. The glands are found all through 

 the life of the animal and upon all parts of the body, but in fewer 

 numbers than upon the pronotum at pupation. Some idea of the 

 abundance of these glands may be gained from the section shown 

 in Fig. i, a section from the pronotum of L. decimlineata at pupa- 

 tion when these unicellular glands are closely crowded and are 

 all ejecting their contents into the space between the old and new 

 cuticula (c 1 , c 2 , r 3 ). 



