438 PHYSIOLOGY OF THE INVERTEBRATA. 



charged by the males are retained in the spermatheca of the 

 queen-bee, and they only escape one by one to fertilise each 

 ovum as it is laid. 



Insects in their most complete character pass through four 

 stages of existence — the ovum, the larva, the pupa, and the 

 imago. In none of these, except the larval stage, does the 

 insect increase in size. Some insects (Aptera) pass only 

 three stages^— the ovum, the "younger stage," and the 

 imago ; and in others the perfect state or imago, is attained 

 without passing through more than two. The ova of insects 

 are usually deposited externally (this deposition in many 

 cases being assisted by an ovipositor), but in some few cases 

 they are hatched in the body of the parent. In the larval 

 stage, the insect moults several times, and after each ecdysis 

 attaining a sudden and rapid increase in size. The larva 

 does not always take the form of a grub or maggot ; for in 

 the Aptera, Hemiptera, and Orthoptera, it assumes a good 

 deal of the appearance of the perfect insect. In this imperfect 

 metamorphosis it changes its skin as the maggots do, and it 

 does not assume a different form for the pupal stage. In 

 the Biptera, Symenoptera, Nev/roptera, Lepidoptera, and 

 Coleoptera, the larvae, on their last change of skin, assume 

 the pupal stage, in which they remain dormant until the last 

 change takes place, when they come out as perfect insects. 

 In some cases the pupae remain on or in the earth ; while in 

 others, cocoons or cases are made by the larvae in which they 

 pass the pupal stage. 



In concluding our remarks concerning the modes of re- 

 production and development in the Insecta, it may be stated 

 that a ~very full account of the genital organs and their 

 countless modifications in the various orders, genera, &c., are 

 given in Von Siebold's Anatomy of the Invertebrata* to which 

 the reader is referred. 



* This is one of the best books on the subject ever written, and is still 

 indispensable to the biologist. 



