INSECTS. 73 



from which the branches issue, or the air-tubes pass at Qsce from 

 each stigma to the organs, but even here also form lateral com- 

 munications. 



Reproductive organs. In all insects sexual reproduction takes 

 place, with the exception of the nurse-formation occurring in 

 summer in the Aphides. The females, the sexual organs of which 

 are sometimes abortive (the so-called neuters), have two ovaries, 

 short oviducts, and a vagina with peculiar appendages, namely, 

 the copulative pouch, which receives the seminal filaments from 

 the penis of the male during copulation, the seminal receptacles, 

 which often constitute two large spiral tubes, into which the se- 

 minal filaments afterwards wander, and in which they remain for 

 months, the cement-organ, that is to say, glands immediately in 

 front of the sexual aperture, which furnish the external shell of 

 the egg. 



The males have two tubular or racemose testes, two seminal 

 ducts, often furnished with lateral seminal vesicles, and which 

 coalesce to form one seminal duct, with the reception of two 

 glandular tubes. The seminal filaments are usually capillary, 

 sometimes inclosed in firm sacs (spermatophora). Copulation is 

 often the principal purpose of these animals. The entrance of the 

 vagina is frequently beset with horny bands, and other horny 

 appendages which we denominate the ovipositor} when it is den- 

 ticulated externally, and intended for boring, it is called the saw, 

 in a simple and finely pointed form the terebra, and when it 

 stands in connection with a poison-gland, the sting. 



The males are generally smaller, brighter in colour, and fur- 

 nished with various excrescences, sucking discs, &c. The care of 

 the eggs is entirely left to the females, and sometimes extends so 

 far, that the latter assist the brood in their pupation and ex- 

 clusion. 



The eggs, which are of very various forms, but usually oval or 

 cylindrical, often have different appendages, and are also cemented 

 together ; a granular yelk, germinal vesicle, and germ-spot are 

 wanting in the mature eggs. The development in the egg takes 

 place in accordance with the type described under the Arti- 

 culata. 



1. The creature excluded from the egg is but rarely similar to 

 its parents in form, &c. ; it generally becomes like its parents only 

 in consequence of several changes of skin, after the last of which 

 only it propagates. The envelopes usually burst in the neigh- 



