INSECT A. 507 



The ovum is invested in a tough shell or chorion produced by the 

 epithelium of the ovarian tube. This chorion is pierced by at least one 

 pore or micropyle through which the spermatozoon enters as the ovum 

 passes down the vagina. A fold of the blastoderm over the ventral plate 

 gives origin in the majority of Insecta to two embryonic membranes, an 

 outer serous envelope and an inner amnion. 



A few Insecta are viviparous. The Tachinae, some Oestridae among 

 Diptera, some Staphylinidae among Coleoptera, the Strepsiptera (note p. 511) 

 produce larvae ; the Ptipipara among Diptera pupae which are nourished 

 in the larval condition by a special gland ; and certain generations Q{ Aphidae 

 among Homoptera, young which closely resemble the parent. Parthenogenesis 

 occurs normally among certain groups, and then the female possesses either 

 complete sexual organs (Psyche, Solenobia (two sp.) among Lepidoptera\ Coc- 

 cidae among Homoptera ; certain Cynipidae^, and Tenthredinidae among 

 Hymenoptera ; Gastrophysa Raphani among Coleoptera 1 ), or the ovaria and 

 ducts are perfect but accessory organs wanting (Aphidae among Homoptera). 

 Voluntary parthenogenesis occurs in the queen Bees, Humble Bees, and 

 Wasps, among Hymenoptera; occasional parthenogenesis among Lepidoptera, 

 as also in the worker Bee, Wasp and Ant which are dimorphic females 

 with rudimentary ovaria and ducts. The offspring produced are of the 

 male sex in the Bees, Wasps (and Ants?) ; of the female in Psyche, Solenobia, 

 Ccccidae, Aphidae and Cynipidae, but males make their appearance at last. 

 In some Tenthredinidae the offspring belong to the male sex, in others 

 to the female, whilst in others again the brood is mixed male and female. 

 The parthenogenetic Aphidae except Chermes and Phylloxera are viviparous. 

 In the Cynipidae there is as a rule only one such generation, but many in 

 Aphidae, Coccidae, Psyche and Solenobia. Paedogenesis or the production 

 of ova by the immature animal is rare, and is in Insecta always partheno- 

 genetic. In the larvae of certain Diptera (Heteropeza, Miastor) the cells 

 of the genital rudiment develope into larvae which feed on the parent 

 and burst their way out through the cuticle ; and the pupa of a Chironomus 

 (Diptera) produces ova at a very early period which are laid just before 

 or as soon as the imago becomes free. The Strepsiptera are said by Von 

 Siebold to afford another instance of its occurrence. 



The Insect at birth is always minute in size. Growth is accompanied 

 by moults or ecdyses, and in many instances by changes, variable in 

 degree, in the structure of the antennae, eyes, mouth-parts, body-somites, 

 and internal organs, making a metamorphosis more or less complete. 

 Accordingly Insecta may be described as Ametabola, Hemimetabola, and 

 Metabola (or Holo-metabola-). In the Ametabola, the young insect or 



1 It is not stated if the organs are complete in these instances. 



3 These terms are defined here in a sense which differs somewhat from that generally given to 



