NORTH AMERICAN ODONATA. 187 



tissues of water plants, and also to steady the abdomen during ovi- 

 position. In the remaining Gomphinse and in the Libellulidae, the 

 vulvar lamina, although often large and variously shaped, does not 

 serve for the insertion of eggs within plants, and there are no genital 

 valves. 



The ova (unfertilized) are elongated, and as they lie with their 

 long axes parallel to the long axis of the ovarian tubes, their two 

 extremities may be definitely distinguished as anter'or and posterior 

 poles. The anterior pole is that which is towards the head of the 

 mother ; it corresponds to the head end of the embryo in late, though 

 not in early, embryonic life. The other pole is the posterior, is di- 

 rected towards the tail end of the mother and corresponds to the tail 

 end of late embryonic life. The micropyle is situated at the anterior 

 pole. 



Secondary sexual differences are of size, of structure and of color. 

 In some groups the males, in others the females are larger, as may be 

 seen from the dimensions given for the species in Part II. Structural 

 differences are found in differences of venation of the post-costal 

 space in some Calopteryginse (Hetceriua) ; in the shape of the pro- 

 thorax and of the tenth abdominal segment in many Agrioninse; in 

 the shape of the occiput, the relative development of the femoral 

 spines, and the possession of spines on the vertex by the female, in 

 many Gomphinse ; in the replacement of the tibial spines by knobs 

 in the males, but not in the females of Cordnleg aster ; in the posses- 

 sion of auricles and an excavated anal border of the hind wings by 

 the males of Cordulinse and many Aeschnidie. When the colors of 

 male and female are different, those of the male are usually brighter. 

 The pattern of coloring both of abdomen and of wings may be quite 

 different in the two sexes ; in such cases, that of the teneral male is 

 more like that of the adult female than is that of the adult male. 

 From various considerations Walsh concluded that in many \e.g. the 

 Calopterygine Lak and Hetnerina and some Gomphus'] though by no 

 means in all Odonatous groups there is a great overplus of males 

 (Proc. Ent. Soc Phila. ii, p. 223). 



Dimorphic females exist in the Libelluline genus Neurothemis, 

 where the heteromorphus female has a more open venation than the 

 male and the isomorphous female (Brauer), and in the Agrionine 

 genera Ceratura, Anomalagrion, Ischnura (see Part II) and Agrioc- 

 uemis, where the coloring of the lieteromorphous female is quite dif- 

 ferent from that of the male and normal female (Selys). 



TEANS. AM. ENT. SOC. XX. AUGUST. 1893. 



