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ILLINOIS BIOLOGICAL MONOGRAPHS 



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this suture is most conveniently expressed in terms of its ratio to the length 

 of the front. The quotient obtained by dividing the length of this sclerite 

 by that of the epicranial stem will be referred to as the epicranial index 

 and expressed by F/Ep. The great majority of lepidopterous larvae have a 

 fairly long epicranial stem, Types 4 and 5 predominating. Type 5, with the 

 epicranial stem longer than the front, occurs more frequently than Type 4, 

 with the front exceeding the stem in length, in the Noctuidae and their 

 allies, as well as in the Sphingidae and Rhopalocera. The average epi- 

 cranial index normally found in the Noctuidae is about 0.7, the stem being 

 somewhat longer than the front. Within the great superfamily Noctuoidea, 

 the markedly short epicranial stem occurs only in certain genera of the 

 Noctuidae. This condition is very frequently seen, nevertheless, since 



Hypothetical figures showing the relation of the front and epicranial stem, adf, adfrontal 

 sclerite; cc, cervacoria; ea, epicranial arm; es, epicranial stem;/, front. 



many of our commonest and most economically important noctuid larvae 

 present this type of head. The infrequent occurrence of the reduced epi- 

 cranial stem in the Noctuidae and allied families indicates that this is a 

 specialized condition, at least in this group. 



Moreover, wherever found in lepidopterous larvae, the short epicranial 

 stem is associated with a specialized feeding-habit, that is, a habit other 

 than the usual leaf-eating one, which we may reasonably attribute to the 

 ancestral lepidopterous larvae, on the basis of its general occurrence in 

 existing forms. Similarly, the parasitic life of certain of the family Orys- 

 sidae is to be considered as a specialized one, since the larvae of the horn- 

 tails are typically borers. Leaf-mining larvae whether coleopterous, dip- 

 terous, or lepidopterous furnish an instance of specialized habit. The root- 

 boring habit of the larvae of Hepialus is to be regarded as a biological 

 specialization, altho their adults are structurally generalized. The larva 

 of the noctuid, Epheiixis lubricalis, feeds upon dry dead-wood, that of 

 Scolecocampa liburna on moist dead-wood. Various cut-worms are sub- 

 terranean to a greater or less extent. Each of these modes of feeding 



