19 lo] .4 Structural Study of Some Caterpillars 97 



It varies considerably in the position of its setae, and is flat, not 

 easily distorted, and easily studied under the microscope. The 

 setae which appear on the margin in figure 1 2 have often migrated 

 inward a distance on the surface of the labrum, and in different 

 groups it is different setae that have migrated. x\s is often the 

 case these diff'erences define smaller groups in the butterflies, 

 than in the moths. For example, vi has migrated up toward ii 

 in the Sphingidae, in Apatelodes, which shows other resemblances 

 to the Sphingidae also; and in a part only of the Nymphalinae 

 among the butterflies. (Figs. 14 to 18). iii migrates inward in 

 the Pierids. Another striking arrangement occurs in the Skip- 

 ])ers, as shown by Figs. 23 and 24. 



The mandibles are the heavy jaw^s, and are the only ones used 

 for biting. They bear tw^o setse on the outer side and the edge is 

 more or less notched, the notches of the two sides fitting into 

 each other. Taxonomically they seem more characteristic of 

 genera than of larger groups. The position of the distal seta 

 far out on the scrobe, as shown in the figure, is typical of the 

 Sphingidse. The Arctiidas show interesting variations in struc- 

 ture of the mandibles. 



The labium forms the middle of the hind margin of the 

 mouth. It is roughly cylindrical. The basal part is formed by 

 a long, usually lightly-chitinized joint, the mentum, which bears, 

 two setae near its middle. Resting on that is the heavy ring- 

 shaped sclerite, a, whose set^ are shown in the figure as free 

 in the membrane distad of the sclerite. This is often the case. 

 The tip of the labium is retractile, and therefore largely formed 

 of membrane (see Fig. 3). Projecting from its center is the 

 cylindrical or flattened spinneret, which is of membrane, strength- 

 ened by three chitinous bands. Surrounding the base of the 

 spinneret is the ring, c, composed of an inner and an outer semi- 

 circular sclerite. It bears a puncture at each side, and may 

 be either very wide and heavy as in the Psychidae, or reduced 

 to a narrow ring about the puncture, as in typical Noctuidac. 

 The labial palpi spring from an area of membrane on each side 

 of c. There is one large and one minute joint, each bearing a 

 seta. In Catocala there is also a rudimentary basal joint. The 

 remaining sclerite, b, of the labium forms a semicircle about the 

 base of the palp. It bears two punctures at its ventral end. 



The siibmentum is divided into a pair of triangular sclerites 

 at the base of the labium. These are usually separated by the 

 base of the mentum. (Compare Figs. 2 and 25). 



