AMERICAN NEUROPTERA. 129 



Of the various groups of labia whicli liave been considered here, 

 certain adult Zygoptera alone show the inner and outer laminae 

 separated by a distinct suture (PI. VII, Figs. 6 and 9 b). As these 

 species also exhibit a considerable cleft in the ligula, together with 

 evidence that at one time this cleft e.xteuded to the base of the ligula ; 

 and as the movable hook on the lateral lobe still retains much of its 

 palpus-like shape, it seems reasonable to conclude that in these spe- 

 cies we have the most generalized and primitive of the labia of the 

 Odonata. 



The Gomphinse and ^Eschninse are considerably more specialized, 

 having lost all trace of the suture dividing the inner from the outer 

 laminjB, while the cleft, with the exception of a mere hint in one or 

 two species, is absent in the entire family. The Libellulidae are the 

 most specialized of all, the mentum and ligula being completely con- 

 solidated without the slightest evidence of a cleft, and the lateral 

 lobes having completely lost their palpus like character. It is from 

 the greatly developed and hollowed out lateral lobes of this family 

 that the comparison of the labium to a mask takes its origin. 



The nymphal stages of Nearobasis chiaensls and Calopteryx macu- 

 lata (PI. V, Figs. 3 and 5) show the ligula and mentum cleft to a 

 remarkable depth. It has semetimes been inferred from this that 

 in these species we find the most primitive type of labium ; this 

 theory is, however, rendered untenable by the fact that the depth of 

 the cleft is much gi'eater in the full-grown nymph, in proportion to 

 its size, than in the earlier stages. It seems more probable, espe- 

 cially in view of the fact that greater specialization of labium always 

 exists in the nymph than in the adult, that this unusual depth of 

 cleft is simply a development to better adapt the nymph to its man- 

 ner of life. Possibly this hole may be used as a place of disposal of 

 rejected portions of its prey. 



Anax Junius and Lestes uncata are specialized along widelv dif- 

 ferent lines. As already shown the tendency of Ledes through- 

 out its nymphal life is to the development of an increasing 

 number of teeth on both lateral lobes and ligula, and to the devel 

 opment of raptorial setse on lateral lobe and mentum. In Ana.r, on 

 the contrary, the tendency is toward greater simplicity ; the number 

 of teeth constantly diminishes; and it has heretofore been believed 

 that no setse were ever present. It will, however, be noticed that 



TEANS. AM. KNT. SOC. XXX. (17) JUNE. 1904. 



