xi PHYLUM ARTHROPODA 647 



this median oviduct, or opening close to it, are receptacula seminis 

 (rs.) and colleterial or cement-glands (sd.). Sometimes there is a 

 copulatory sac, or bnrsa copulatrix (nva.). In the male the paired 

 testes (B, t.) vary greatly in form : sometimes each is a long narrow 

 tube ; sometimes several such tubes combine to form the testis ; or 

 it may be of more compact rounded form and entire or lobed. 

 Each testis has a slender duct or vas deferens (B, vd), the two 

 vasa deferentia uniting to form a median ejaculatory duct. A 

 vesicula seminalis is appended to each vas deferens or to the 

 ejaculatory duct. Accessory glands, opening into the vas deferens 

 or the ejaculatory duct, secrete cementing material for uniting the 

 sperms into masses, the spermatophores. In most instances the 

 eggs are laid shortly after their fertilisation, only a comparatively 

 few forms, such as the Aphides or Plant-lice, many Diptera, and 

 some Coleoptera, being viviparous. Some Insects, such as the 

 Aphides and the Bees and Wasps, as well as some Lepidoptera 

 and Neuroptera, present us with the unusual phenomenon of 

 parthenogenesis ; i.e., ova are formed, as in ordinary female insects, 

 in organs corresponding to the ovaries of the latter, and are 

 developed without fertilisation. In the case of the Aphides, an 

 autumn generation of completely-developed males and females is 

 followed by a spring generation consisting entirely of females ; 

 these are both parthenogenetic and viviparous. In the Bees, 

 the workers (imperfectly developed females) occasionally produce 

 ova which, without fertilisation, develop into drones (males). In 

 one or two groups, including the Scale-Insects (Coccidce) and 

 Gall-Insects (Cynipidce), males are never developed, so that repro- 

 duction is exclusively parthenogenetic. Pcedogenesis accompanies 

 parthenogenesis in certain Diptera ; i.e., the larvce produce ova and 

 embryos without impregnation. 



The eggs when laid are protected from injury by a number of 

 methods ; they may be firmly fixed to the substratum, buried in 

 the earth, or laid in the interior of certain plants or even of 

 animals. The deposition of the eggs, by means of ovipositors, in 

 the leaves or other parts of plants gives rise to swellings the 

 so-called galls, in the interior of which the young Insects live. 

 In the case of many Insects the eggs are enclosed in a cocoon ; in 

 others they are surrounded by gelatinous or waxy material. The 

 eggs are, for the most part, of relatively considerable size. In form 

 they vary, but the long oval prevails in most instances. The ripe 

 egg is enclosed in two egg-membranes an inner, the vitelline 

 membrane, produced by the egg itself, and an outer, the chorion, 

 formed from the follicle-cells. The chorion, which usually exhibits 

 a more or less elaborate pattern, has one or more apertures or 

 micropyles for the entry of the sperm. The contents are dis- 

 tinguishable into two layers a superficial, consisting of proto- 

 plasm, and a central, of nutrient yolk. 



T T 2 



