62 ENTOMOLOGY 



Collembola, the embryo has paired abdominal limbs, and those of the 

 first abdominal segment eventually unite to form the peculiar ventral 

 tube (Fig. 13) of these insects, while those of the fourth segment form 

 the characteristic leaping organ, or furcula, and those of the third, the 

 tenaculum. 



Cerci. In many of the more generalized insects the abdomen bears 

 at its extremity a pair of appendages termed cerci. These occur in 

 both sexes and are frequently long and multiarticulate, as in Thysanura 

 (Figs. 78, 10, n), Plecoptera (Fig. 19) and Ephemerida (Figs. 20, ;86) 

 though shorter in cockroaches and reduced to a single sclerite in Locus- 

 tidae (Fig. 89). The paired cerci, or cercopoda of Packard, are usually 

 though not always associated with the eleventh abdominal segment and 

 are homologous with legs, as Ayers has found in (Ecanthus and Wheeler 

 in Xiphidium. As to their function, the cerci of Thysanura are tactile, 

 and those of the cockroach olfactory, while the cerci of male Locustidae 

 often serve to hold the female during copulation. 



The so-called "median cercus" or "filum terminale" of Thysanura 

 (Figs, ii, 78) and Ephemerida (Fig. 86) resembles the true cerci of 

 these insects in being multiarticulate and usually long, and in having the 

 same function; but differs from these morphologically in arising as a 

 median dorsal prolongation of the eleventh abdominal segment; being 

 therefore not equivalent to one of the paired segmental appendages. 

 For this median filament the term pseudocercus is appropriate. 



Extremity of Abdomen. Various modifications of the terminal 

 segments of the abdomen occur for the purposes of defecation and 

 especially reproduction. The anus, dorsal in position, opens always 

 through the last segment and is often shielded above by a suranal plate 

 and on each side by a lateral plate. The genital orifice is always ventral 

 in position and occurs commonly on the ninth abdominal segment, 

 though there is some variation in this respect. The external, or 

 accessory, organs of reproduction are termed the genitalia. 



Female Genitalia. In Neuroptera, Coleoptera, Lepidoptera and 

 Diptera the vagina simply opens to the exterior or else with the anus 

 into a common chamber, or cloaca. Often, as in Cerambyx (Fig. 79) 

 and Dasyneura (Fig. 80) the attenuated distal segments of the abdomen 

 serve the purpose of an ovipositor; thus in Itonididae, the terminal 

 segments, telescoped into one another when not in use, form when 

 extruded a lash-like organ exceeding frequently the remainder of the 

 body in length. 



A true ovipositor occurs in Thysanura, Orthoptera, Odonata, Hemip- 



