28 UNITED STATES NATIONAL MUSEUM BULLETIN 2 76 



attach themselves by their crochets to a flat mat of silk which has been 

 spun on some support, but this is as close as they come to building a 

 tent. 



M. constrictum and M. tigris are two species having similar tent- 

 building habits. Both build a new "tent" each time they molt, but 

 after molting they abandon it and cluster on branches in a manner 

 similar to M. disstria. These molting tents are quite snaall, the largest, 

 last instar ones usually being not more than 2 or 3 inches in diameter 

 (tents this large are rare), and spun quite closely to the branches or 

 leaves so there is practically no space on the inside (fig. 334). The 

 larvae never rest inside these small tents as do the species which build 

 large tents. Larvae of M. constrictum have been found resting on the 

 outside of tents of other species, such as M. californicum and its sub- 

 species, but no constrictum larvae were ever found inside these tents. 

 The tents of both constrictum and tigris usually are inconspicuous, and 

 this, no doubt, is one of the major reasons for the belief that constrictum 

 is not very abundant, and for the fact that tigris has not been recognized 

 in the literature since it was originally described in 1902 (except for 

 recent reports in the "Cooperative Economic Insect Report," 1963 and 

 1964, where it was referred to as ^'Malacosoma sp., probably texanum''). 



Variations in the behavior and reactions of diflferent species are 

 related to whether or not they construct tents and live in them. Studies 

 made by Sullivan and Wellington (1953) have shown that all instars 

 of M. disstria become photonegative when they are sufficiently heated. 

 In contrast, they found the first three instars of M. americanum and 

 Af . [californicum] pluviale showed the same reactions when heated suffi- 

 ciently, but the fourth, fifth, and sixth instars showed a photopositive 

 reaction. They believe this photopositive reaction of the larger larvae 

 of the tentbuilders to have survival value under ordinary conditions, 

 since the larvae leave the tent when it becomes too hot and rest outside 

 the tent where temperatures normally are cooler. M. disstria^s photo- 

 negative reaction also has survival value since larvae move to the 

 shaded sides of branches when it becomes too hot. 



In this respect there is an interesting difference between the be- 

 havior or reaction of M. disstria larvae and that of M. tigris. In 1962 

 near Inks Lake State Park, Burnet County, Texas, both species were 

 common and causing heavy defoliation of the oaks. M. disstria builds 

 no tent and M. tigris builds only small molting tents, and both rest 

 on branches when not feeding. In this locality, however, mature 

 disstria larvae invariably were found resting on the shady side of 

 branches or trunks, but mature tigris larvae were found resting in many 

 olaces with no apparent preference for sun or shade. This difference 

 "or was noticed about noon on a rather warm day. The exact 



