THE EXTERNAL STRUCTURE OF THE LEPfDOPTEROUS LARVA. 39 



Acronicta lepurina (slightly in first skin), Cuspidia megacephala (in 

 fourth skin, very noticeable and long, almost secondary hairs), 

 Pharetra euphorbiae var. myricae (first to third skins), Pachnobia leaco- 

 grapha (weak in first skin, no trace after), Triphaena pronuba, T. comes, 

 T. fimbria, T. ianthina (just traceable in first skin, then absent), 

 Peridroma saucia and Agrotis puta (first skin, very small), Dianthoecia 

 carpophaga (to full-grown, very long), Taeniocampa miniosa (large and 

 distinct in first *skin, only traces after), T. gracilis (very fine, black, in 

 first skin, no trace after), T.pulverulenta (strongly marked throughout), 

 Calocampa exoleta (in first, no trace in fourth, skin), Aporophyla 

 australis (absent, or exceedingly fine in first skin), Galymnia ajfinis 

 (strong in first, small in third, skin, no trace later), Polia chi (slight 

 traces in first skin), Dicycla oo (strongly marked throughout), Plusia 

 festucae (present in third skin). Papilionides : Zepkyrus quercus (strong 

 when, and not until, full-fed), Aglais urticae (strong, in early stages). 

 Since the observations, on which this list is compiled, were made off- 

 hand, and when studying other characters presented by the larvae, it 

 can be readily understood how common an occurrence is the presence 

 of this pile in lepidopterous larva?. 



Bacot says : Most of the Noctuids lose the character very early, yet 

 in some it persists strongly throughout the larval life. Dianthoecia 

 carpophaga exhibits it from the youngest to adult stage, yet adult 

 D. cucnbali shows no trace of it. T. pidverulenta retains, but T. miniosa 

 soon loses, it. 



Scudder believes that " the use of this clothing is tolerably clear, 

 since this pile must prevent the too rapid evaporation of the heat from 

 the surface of the body, for, although caterpillars are classed among 

 the cold-blooded animals, they, nevertheless, have an internal heat 

 above that of the surrounding atmosphere, which originates from the 

 activities of the organs and the respiratory functions, and which they 

 would lose more rapidly but for this investing pile." 



On the dorsum of the thoracic (and more rarely the abdominal) 

 segments of the larva, a hard chitinous shield is found. This is par- 

 ticularly noticeable in all wood-boring larva?, such as those of the 

 Cossids, Hepialids and Sesiids, as well as in Crambids, Tortricids, and 

 many Noctuids and Tineids. It is, however, more general and most 

 marked on the pro-thorax, and hence it is often spoken of as the pro- 

 thoracic shield. Since this structure is equally well-developed in the 

 larvae of the Cerambycidae and other Coleopterous larva? which also 

 bore into hard substances, it appears probable that this hard chitinous 

 plate serves to protect the head, and parts of the body underlying the 

 shield, from injury. Its appearance, too, in larva? belonging not only 

 to different families of the Lepidoptera, but also, to different orders, 

 suggests that it has been developed in response to the external stimulus 

 supplied by continual friction, an excess of chitin having been deposited 

 (or developed) by the hypodermal cells of the tergal arch of the pro- 

 thoracic segment. It is not unusual to find the shield, in some 

 form of decadence, in larva? which now feed fully exposed, especially 

 in certain Noctuids, and occasionally the shield is present in the first 

 larval stage, but lost in the later ones. These occurrences generally take 

 place in larva? some of whose allies have, or had, boring habits. The 

 value of this shield to boring larva? for leverage purposes must also be 

 very great, since it gives a solid fulcrum for the head. The excessive 



