424 



on the dorsum near the caudal margin, similar in position to that of 

 the nymph, and is, as a rule, indistinct unless the head is specially pre- 

 pared in caustic potash. The furrow begins near the caudal margin 

 of the dorsum, extends cephalad a short distance, forks, and extends 

 latero-cephalad, caudad of the ocelli, to the margins of the compound 

 eyes (Fig. 32, epcs). It can not be traced to the occipital foramen, 

 but the homology of the furrow as a whole can not be doubted. There 

 are three ocelli (o) cephalad of the Y, which are sometimes elevated 

 above the surface of the head forming the so-called ocellar area. A 

 furrow extends cephalad from the angle of the Y between the lateral 

 ocelli and forks just caudad of the median ocellus. This furrow is 

 present in many orders of insects, but its true homology is not known. 

 The front includes that portion of the dorsal aspect cephalad of the 

 epicranial Y, between the compound eyes and cephalo-ventrad to the 

 fronto-clypeal suture (Fig. 32, f). Cephalad of the median ocellus 

 there is always a short, deep, transverse furrow which, although pres- 

 ent in most Odonata, must not be mistaken for a suture. The fronto- 

 clypeal suture does not reach the margins of the compound eyes on 

 either side. There is always a polished area on each side of the clypeus, 

 which is a portion of the gena (Fig. 32, gn). The clypeus (Fig. 

 32, cly) extends ventrad of the fronto-clypeal suture and is divided 

 into two parts by a transverse median ridge. The dorsal part, often 

 dark and heavily chitinized, is the postclypeus ; the ventral one, more 

 weakly chitinized and often wrinkled, is the anteclypeus. The clypeo- 

 labral suture separates the clypeus from the sclerite ventrad of it, the 

 labrum (Fig. 32, Ibr). This sclerite is only slightly bilobed in most 

 species of Zygoptera, the ventral margin is directed caudad, and the 

 lateral margins are convexly rounded. Laterad of the bases of the 

 mandibles, which lie at either side of the clypeus and labrum, there 

 are small semi-ovate sclerites, the trochantins of the mandibles (Fig. 

 32, tm). The fronto-genal sutures are indistinct, but are represented 

 by furrows extending from the dorsal articulations of the mandibles 

 to the antennal fossae and laterad to the compound eyes. That por- 

 tion of the head on the dorsum and caudad of the arms of the epi- 

 cranial Y, is the vertex (Fig. 32, vx), but it is not separated by a dis- 

 tinct suture from the occiput, which occupies the dorsal half or third 

 of the caudal aspect (Fig. 30, oct). The postgenae, which occupy the 

 ventral half of this aspect are separated from the genae by the oblique 

 ridge mentioned above. There is another ridge starting from the 

 ventral condyle of the mandibles (Fig. 30, ocr) but extending dorsad 

 instead of latero-dorsad. This ridge disappears near the middle of 

 the head. 



