IO BULLETIN OF THE BUREAU OF FISHERIES. 



several larvae removed from similar stems were placed. In the outdoor experiments the 

 rafts were made of freshly cut stems about 2 feet long and were left to float freely in 

 an infested portion of a pool where similar larvae could be taken at any time during the 

 year. These stems were observed at intervals for two months, and none showed any signs 

 of the presence of the larvae. In this relatively short period they showed but slight 

 signs of decay and practically no accumulation of diatoms. 



In the laboratory experiments with sections of similar stems in a more advanced 

 stage of decay only such stems as had already been infested were used. It was soon found 

 that the larvae would readily accept these stems, which they usually entered by creeping 

 into the openings at the ends. The sections cut to fit into a Syracuse watch glass could 

 ordinarily be entered from the ends and were usually short enough to enable the larvae to 

 maintain their water current without penetrating the epidermis. In order to make it 

 necessary for the larvae to penetrate the epidermis, the cut ends were coated with melted 

 paraffin. The result in many cases was that they simply crawled under the stems and 

 spun their silken tubes, fastening them to the stem above and the glass below. In only 

 one instance did a larva penetrate the stem from the side. In this case the opening was 

 rough and jagged in outline and was located near one of the lower corners of the stem, 

 where it seems fair to assume that the larva might have gained some advantage (by 

 catching its posterior end under the edge of the stem) from the sharpness of the angle 

 that would in a way compensate for its light weight in bringing pressure to bear on the 

 mouth parts. This seems especially possible when it is observed that the claws of the pos- 

 terior prolegs point forward and are capable of holding the posterior end of the body in 

 place while the muscles of the body are used in flexing the body and holding the mouth 

 parts of the larvae in contact with the epidermis. 



Observation on a series of stems selected at random from among a considerable 

 number dipped up from the bottom of a pool where the larvae were abundant showed 

 a greater number of larvae near the ends of the stems. In some cases a larva was so 

 located that one end of its tube opened at the end of the stem and the other by an 

 opening bored through the epidermis. Several stems were found to have openings 

 along their entire length, but all were confined to what had been the inner or upper 

 surface of the leaf where the epidermis was thinnest. In other cases the larvae had 

 an opening on one side of the leaf with a long vertical tube leading to its gallery which 

 was on the opposite side, where it opened to the surface through a thickened epidermis. 

 Old Typha stems were occasionally found with larvae located near the broken ends, 

 but in no case was there noticed a larval-made opening penetrating the epidermis. 

 When the Typha stems were tested with a sharp point, the epidermis was found to 

 be very much tougher than that of Sparganium which is most frequently inhabited 

 by the larvae. 



The thickness and texture of the epidermis of a stem is an especially important 

 factor, as the above observations indicate. This, however, is not the only source of 

 evidence, but when considered in connection with the fact that two larval molts out 

 of six examined had one of the lateral teeth of the labial plate broken (fig. 2S) it 

 becomes evident that the larvae exert themselves to the limit in penetrating the various 

 plants in which they construct their galleries. That the larvae more frequently penetrate 

 the epidermis from the inside than from the outside seems to be shown from the greater 



