The Source of Variability 



93 



In primitive caddisflies the larvae have a sclerotized hard dorsal 

 shield on only the first of the three thoracic segments (Fig. 39a). 

 In four completely mirelated phylogenetic lines, the second and third 



pronotum 



Fig. 39. Larvae of two caddisflies. (a) Rhyacophila, having pronotum 

 sclerotized, the succeeding two segments membranous; (b) Htjdropsijche, 

 having pronotum and succeeding two segments sclerotized dorsally. (From 

 Illinois Natural History Survey.) 



thoracic segments also have a sclerotized dorsal shield extremely 

 similar to that of the first segment. Three of the four cases each 

 represent an entire distinctive family, the Hydropsychidae (Fig. 

 39b), Hydroptilidae, and Limnocentropidae, respectively. In the 

 latter family the dorsal shields on the second and third segments 

 are similar to that on the first in so many small details that the 

 influence of a homeotic mutation is strongly suggested. In the 

 fourth case, the genus Ecnomus in the Psychomyiidae, the adults 

 resemble those of a related genus Tinodes so closely that one would 

 expect only a trivial difference in the larvae. Yet the larva of 

 Tinodes has the second and third thoracic segments completely 

 membranous with no indication of an intermediate stage leading 



