43] LEPIDOPTEROUS LARVAE— FRACKER 43 



and vestiture are either absent or indistinguishable in lepidopterous 

 larvae. The mouth parts and antennae of all but a few species are 

 constructed on the same plan and show such limited variation that little 

 use can be made of them. No one has yet discovered characters of much 

 value in the structure of the thoracic legs, so uniform are they through- 

 out the entire order. The vestiture is also entirely different from that 

 of other insects. It is consequently necessary to seek other characters 

 than those with which entomologists are usually familiar and to intro- 

 duce new terms to describe them. 



The structures of value in the classification of these larvae are pri- 

 marily: the head sclerites, head setae, and ocelli; the armature of the 

 body, especially the setae ; the shape of the spiracles ; the number of 

 prolegs and the arrangement of the crochets they bear ; and the presence 

 on the body of humps and gibbosities, eversible glands, or peculiar modi- 

 fications of the usual organization. These will be taken up in order. 



Head parts. — The varying shapes of the head sclerites and positions 

 of the setae are so well shown by Forbes (1910) that they need little 

 attention here. The head capsule consists almost entirely of an epicra- 

 nium divided into three parts by the forked epicranial suture. These 

 parts are the right and left sides and the front. The latter is usually 

 a triangular sclerite on the facial aspect of the head. To its ventral 

 margin is attached the trapezoidal clypeus which supports the labrum. 

 Laterad of each arm of the epicranial suture is a narrow area, the ad- 

 frontal piece (afp, Fig. 78), the exterior indication of the anterior arm 

 of the tentorium. In this region the varying sizes and shapes of the 

 front, the adfrontals, and the labrum, and the location of the setae they 

 bear, are important characters for identification. 



At the point where the caudodorsal part of the head capsule joins 

 either the prothorax or the unchitinized portion of the head, there is a 

 pale triangular dorsal area of thin euticula. The cephalic point of this 

 triangle in all except a few caterpillars is at the caudal end of the epi- 

 cranial suture and its sides are formed by the caudad projecting lobes 

 of the epicranium. This dorsal area is known as the "vertical triangle", 

 or sometimes merel.y the "vertex", although the latter term properly 

 applies to the dorsal part of the head capsule itself. In some leaf- 

 miners the front extends caudodorsad as far as this triangle and the 

 arms of the epicranial suture do not unite to form a stem. The front 

 in such cases is said to be "open". In very small larvae, however, care 

 must be taken in determining this point, for microscopic preparations 

 show the tentorial arms much more distinctly than the epicranial suture, 

 and these internal arms usually do not meet on the vertex. The 

 head setae are numbered according to Dyar's scheme (Figs. 78, 86). 



