14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO7 



line are short or absent. The Protura and Diplura commonly have an 

 internal median ridge along the posterior part of the head. Though 

 Tuxen (1931) in reference to Protura points out that this ridge is an 

 apodemal structure quite unlike the thin line of ecdysis in pterygote 

 insects, it is still possible that its component lamellae may be split 

 apart at ecdysis. 



Ephemeroptera and Plecoptera. — In these two orders the line of 

 exuvial cleavage is plainly marked on the larval head, and has the 

 typical Y-form (fig. 3 A, B, CL). The frontal arms diverge to points 

 between the eyes and the antennae, and may therefore be said to 

 delimit the frons (Fr). At ecdysis the head cuticle splits to the ends 

 of the frontal lines, as shown in the plecopteron Acronenria (C). 



Odonafa. — The cleavage line on the head of larval Odonata is 

 characteristically T-shaped rather than Y-shaped, inasmuch as the 

 frontal arms usually go almost straight laterally (fig. 3D). Accord- 

 ing to Calvert ( 1934) in the first larval exuviae of Anax Junius the 

 transverse fissure does not reach the eyes, as in following ecdyses ; 

 the median fissure, however, extends apparently to the labrum, so that 

 the two clefts form a cross on the top of the head. On the exuviae 

 of subsequent instars the frontal arms of the cleavage line cut through 

 the corneae of the compound eyes, but Calvert says the point at which 

 they enter the corneae moves forward during larval development. 

 While with most dragonflies the exuvial cleft at the last ecdysis cuts 

 the cornea into an upper and a lower section (fig. 3 E, F), Calvert 

 observes that "in at least some Libellulidae it cuts through the side 

 of the head anterior to the eyes and in some members of this family 

 (e. g., Macromia, Plathemis) farther cephalad than in others (e. g., 

 Libellula, Sympetrimi) ."" Henriksen (1932) says the cleavage line is 

 easily seen on the eye of an odonate larva, but the writer has dis- 

 covered no trace of it on the eyes of specimens examined before 

 ecdysis. 



Dermaptera. — The Y-line of cuticular cleavage on the head in this 

 order is as plainly marked in the adult (fig. 4 A, B) as in the nymph. 

 In Anisolabis mm-itima (A) the arms of the cleavage line curve 

 outward and forward to the mesal angles of the eyes, where they are 

 met by the lines of internal ridges extending upward laterad of the 

 antennal sockets, but the writer has no examples of the splits as 

 formed at ecdysis in this species. In Forficula auricularia also the 

 arms of the cleavage line go to the compound eyes (B), and at ecdysis, 

 as noted also by Henriksen (1932), the cuticular splits bisect the 

 corneae (C). The frontal apotome (frapt) of the exuviae is then 

 turned forward and downward, widely opening the corneal cleft, 



