244 Arthropoda. 



single duct,* wliioli opens in a similar position to tlie vagina of the 

 female. Each, of the vasa deferentia widens posteriorly to form a 

 vesicula seminalis'. Special glandular appendages frequently- 

 open into these ducts, or into their common portion. There is a more 

 or less complicated copulatory organ, an evagination of the 

 hody-wall through which the terminal portion of the seminal duct is 

 continued, and capable of partial or complete retraction when not 

 in use ; in many, it may possess hard chitinous portions, and lies 

 hidden within the cloaca, from which it may be protruded during 

 copulation. 



A fairly marked sexual dimorphism ocom-s very often in Insecta, 

 due largely to tlie different parts played by tte male and female in repro- 

 duction. Frequently the males possess apparatus wHcli is wanting in the 

 females, or certain portions of the body are specially developed ; for example, 

 the large mandibles of the Stag-beetle, the huge eyes of the male Honey-bee, 

 the well-developed antennse of the male Oock-chafers and many Butterflies, 

 the broad front feet of male Water-beetles ; such developments, if they are 

 in any way explicable, are attributable to the struggles carried on by the 

 males for the possession of the female (Stag-beetle) ; or, in the case of special 

 prominence of sensory organs, they result from the needs of the male in seeking 

 for the less active female, or the parts serve as organs of retention dtu-ing copula- 

 tion (Water-beetles). More rarely some portion of the body of the female is 



specially developed; in the female 

 2 2 3 Nut- weevil {Balaninus nucum), the 



proboscis is longer than in the male, 

 as it is used to gnaw thi-ough the 

 yoimg nuts, in which the eggs are 

 deposited. Not infrequently the 

 sexes differ in. size, the pre- 

 ponderence beiag usually on the 



side of the female ; this may be 

 Mg. 208. Females of three alHed species of ^^^ ^j j ^ ^j^^ ^ bulk of 



Geometria£e (1 Hibernia progemmana, 2 H. mi 



aurantiaria, 3 H. defoUaria), to show the sue- the ova. There are very often, 

 cessive stages in the degeneration of the wings. — also, differences in colour and 

 After Eatzeburg. form, but these, like many plastic 



differences (e.g., those between the 

 male and female Oryctes), are, as a i-ule, not apparently capable of explanation. 

 As already mentioned, the male is usually more active than the female, 

 and in correlation with this, sexual dimorphism may be carried very far, 

 various parts of the body of the female may be considerably modified or 

 atrophied. In not a few Lepidoptera for instance, the wings of the female are 

 considerably shortened, so that they have become useless as organs of flight, or 

 "they are quite rudimentary, or have vanished altogether ; in some forms, indeed, 

 degeneration goes still further, the legs are feeble, or are not developed, so that 

 the animal sinks into a maggot-like state, which is as different from the male as 

 possible. The converse, when the female has greater activity, may also occur, 

 although more rarely; Fig. 209 for instance, shows a species of small Gall- 

 fly (Blastophaga grossorum), which lives as a larva in the tiny seeds of the 



* This duct (like the vagina) may be absent (Ephemera, and a few others), and 

 the vasa deferentia then open separately. 



