66 Bulletin American Museum of Natural History [Vol. XLII 



The common habitat of all the species seems to be in small pools, 

 either at the water's edge in the mud or in the Spirogyra farther out. 

 P. nebulosus may be taken at almost all times of the year, depending in 

 the winter upon warm spells. October 17 of one year proved to be the 

 most profitable day encountered for the procuring of the adults of this 

 species, when as many as fifty were taken from the roots of the vegeta- 

 tion bordering the shores of Dwyer's Pond. 



The eggs, which vary in number and size in the different species, 

 are usually enclosed in an outer covering of silk or cocoon in addition 

 to the usual inner layer and are particularly characterized by the ribbon- 

 like filament, which only the genus Hydrobius approaches. The cocoons 

 are found attached to all kinds of objects but usually at the surface of 

 the water, more frequently to blades of grass or Spirogyra, with its rib- 

 bon stretched out over the surface. They are laid most abundantly 

 during May but are found from April to August in the field. Indoors 

 they are apt to be laid any month in the year. Ten egg-cases were made 

 by a single specimen of Philydrus nebulosus during February and some 

 of them were placed below the surface of the water. The time required 

 for hatching is from six to nine days. 



The larvae may be procured during the summer months by strenu- 

 ously washing the banks of pools where the egg-cases or adults are found. 

 They seem to burrow in the mud near the water's edge, while some 

 repose on the under side of stones as far from the water as it is moist. 

 In their method of walking they resemble very closely the eruciform type, 

 as their motion is rhythmic. The true legs move first, the inconspicuous 

 prolegs next, and finally the end of the abdomen is used. The larval 

 period lasts about two months. 



The pupal stage is quite short, taking up three or four days. This 

 stage was observed indoors either in the aquarium where it was formed 

 or else by taking the mature larva and placing it in a jar with moist 

 earth, where it formed a cell below the surface in the usual manner or 

 pupated on top of the ground. On July 20, 1916, many pupae of P. 

 nebulosus and P. perplexus were found in cells underneath stones or in 

 the moist bank. The pools at this time of the year were gradually dry- 

 ing up and naturally possessed a concentrated array of beetles. 



The progress of coloring in the adult, after emergence, was observed 

 in P. nebulosus. At first the head, thorax, and elytra are very dirty 

 white, nearly brown, and the appendages of the head nearly white, 

 while the legs are tinged a little with brown. The eyes soon get black 

 and the whole body darkens gradually, the thorax getting its color 

 much more slowly than the elytra. 



