INTRODUCTION. 5 



ally has very little effect on them. They vary very much in size, 

 form, and markings. Some are smooth and spherical, others cor- 

 rugated, sculptured, elongated, and tailed. 



The structure and development, as seen under the microscope, form 

 not only a very interesting study, but present many difficult biological 

 problems. Amongst the higher egg-producing animals no egg will 

 develop without male fertilisation, but amongst insects there are many 

 exceptions to this rule. In numerous cases eggs laid by virgin moths 

 have been known to develop. Amongst saw-flies, certain generations 

 of gall-wasps, plant-lice, and others, we have reproduction by virgin 

 females. This law is known as parthenogenesis, or virgin reproduc- 

 tion without the intervention of a male, and a few special peculiarities 

 will be considered under the respective families. 



The morphological structure of the egg is somewhat complicated, 

 more especially when we remember the philosophical writings of 

 Weismann and others. 



The term larva, caterpillar, maggot, or grub is generally applied to 

 insects as they hatch out from the egg, and at this stage they gener- 

 ally differ in form and structure from the perfect insect. On the 

 other hand, the term "nymph" is applied to the creature when it 



'^ 



Fig. 3. — Typical forms of la: 



(From drawings by Mr G. A. Diinlop.) 



bears a very strong resemblance to the perfect insect, as, for example, 

 in the case of mites and Hemipterous insects. 



The larva of a moth, on hatching from the egg, is usually a seg- 

 mented body possessing six true legs. The caterpillar of a saw-fly has 

 six true legs, like the larva of a moth, but has also several additional 

 " prolegs," and consequently is easily distinguished. The maggot of 

 a fly is footless. Fig. 3 represents typical larvse. 



