REVISION MALACOSOMA HUBNER IN NORTH AMERICA 33 



of Malacosoma than whether or not it builds a large tent. Nevertheless, 

 it seems unlikely that the expenditure of energy involved in such com- 

 plex behavior as tentbuilding serves no useful function in those species 

 which construct large tents. It remains to be seen precisely what these 

 function(s) are, and how important they are. 



c 



ocoons 



The cocoons of all species of Malacosoma are constructed somewhat 

 loosely, if they are compared with cocoons such as those of the silk- 

 worm, Bombyx mori (L.). They always consist of a tighter, inner silken 

 case which closely surrounds the pupa, and they sometimes have 

 another outer, loose layer of silk (fig. 309). The presence or absence 

 of this outer layer of silk seems to depend to some extent on the type 

 of site that is chosen for spinning the cocoon. Cocoons spun in tight 

 locations such as cracks and crevices have little or no outer layer, 

 while those found in less protected locations may have leaves or other 

 materials drawn about the cocoon and held in place by the outer 

 layer. Some species, such as M. disstria which normally draws leaves 

 together, almost invariably have an outer layer, while others such as 

 M. californicum fragile usually have none (fig. 310). 



The cocoons of all species are dusted with a yellowish or whitish 

 crystalline powder reported to contain urates (Wigglesworth, 1953).* 

 This powder is very irritating to some people and could possibly 

 furnish a degree of protection from enemies. Essig (1926) reported a 

 rather unusual effect of tent caterpillars, noting that "In California a 

 number of hogs were killed on the ranges of Mendocino County be- 

 cause they ate the cocoons and pupae of this moth [M. disstria] which 

 were very abundant in 1918." This sounds somewhat farfetched, but 

 it could be true since the powder present on the cocoons may cause 

 allergic reactions on some people (see below). 



The powder is present in the malpighian tubules of mature larvae 

 as a thick yellowish or whitish fluid, and is excreted through the anus 

 by the larva after the cocoon is completed. After excreting it, the larva 

 smears it around the inside of the cocoon with its head, forcing it 

 into the spaces between the silk threads of the cocoon. There it dries 

 into a whitish or yellowish caked material which is broken into a fine 

 powder when the cocoon is disturbed. Partially starved or diseased 

 larvae construct flimsy cocoons and often have little or none of the 

 powder found on normal cocoons. 



* The crystals are reported to be calcium oxalate monohydrate by E. Ohnishi 

 et al., 1968. Crystals from cocoons of Malacosoma neustria testacea. Science 160, pp. 

 783-784. 



