1913] Biology of Perl a immarginata Say. 207 



casting of the skin has not been seen in this species. Although 

 adults are rarely absolutely perfect specimens, the percent of 

 individuals lost by inability to complete transformation is 

 exceedingly small. Judging from the fact that no newly 

 emerged insects have been found, it is thought that they must 

 transform during the night, or more probably, in the early 

 hours of the morning. 



Adtdts. 



Characteristics. The adult Perla immarginata, (Fig 5), is 

 uniformly dull brown and much less conspicuous than the 

 nymph. As soon as the insect loses its tracheal gills and 

 gains four well developed wings, it is ready for aerial life. 

 Unlike many adults with this equipment, some of the nymphal 

 tendencies are carried over into this stage. Chief among these 

 is the love for hiding. So great is their shyness that, even at 

 the height of the emerging season, the adults are rarely found 

 in the field. Repeated attempts at sweeping the foliage along 

 the stream have met with little success. Careful searching of 

 the rocky walls of the gorge has occasionally revealed an adult 

 hidden away in a crack or crevice. Similar habits have been 

 noticed in the adults kept in cages. They never rest on the 

 twigs but crawl into hiding under the edge of the stones, or 

 pans, or wherever they can wedge themselves into a tight place. 



Although they avoid day-light, artificial lights attract 

 them at night. They have been found crawling along poles 

 and fences, or in the road under electric lights in the neighbor- 

 hood of streams. 



When disturbed the adults rarely seek escape by flight, but 

 usually by running. Here again we see a nymphal trait, and 

 a characteristic of the group. They are poor flyers and de- 

 pendent upon their legs. Some stone-flies do not fly at all, 

 although provided with fully developed wings. 



Food Habits. A striking difference between the nymph 

 and adult is found in the structure of the mouth and in the 

 food habits. A character long assigned to stone-flies is rudi- 

 mentary mouth parts of a biting type. This is true for Perla 

 immarginata, but not for the entire order. In this species 

 we have the reduction of the strong chitinous mandibles to 

 mere fleshy lobes, (Fig. 1). The very appearance of such an 

 apparatus indicates its uselessness, and examination of the 



