INTRODUCTION. 5 



The abdomen is frequently terminated by various appendages 

 connected with the reproduction of the species. The most con- 

 spicuous of these is generally the ovipositor of the female, which 

 is very various in form, and is sometimes of great length, and 

 not unfrequently bifid or trifid. In the order Eymenoptera it is 

 frequently modified into a sting. 



As a rule, there are two sexes in insects. In some cases the 

 males and females resemble each other closely, but they frequently 

 differ so much that they are liable to be placed in different genera 

 until their relationship is clearly ascertained. Among the social 

 insects (ants, wasps, bees, and termites) the bulk of the com- 

 munity consists of sterile females, called workers, or neuters, of 

 which there are frequently more than one class, which perform all 

 the work of the nest. Among other insects, two forms of males, 

 or two forms of females, are often met with, a phenomenon known 

 as sexual dimorphism. At other times, both sexes will exhibit a 

 similar phenomenon, the species occurring normally in two well- 

 marked forms. When these appear at different times of the year 

 it is called seasonal dimorphism. Apart from this, local or acci- 

 dental variations from the type of a species are of very common 

 occurrence. 



When insects are found paired, the presumption is that they 

 belong to the same species ; but this is not invariably the case. 

 Instances have even been recorded in which a butterfly has been 

 found paired with a moth ; and although such a union would be 

 almost certainly sterile, yet hybrids are not unfrequently produced 

 by the crossing of allied species. Hermaphroditism is a phenomenon 

 of occasional occurrence among insects, and when, as sometimes 

 happens, the sexes of an insect differ considerably, the effect is 

 very striking. Thus, in hybrids of the Common Blue Butterfly 

 (Polyommatus Icarus), one pair of wings may be blue, and the other 

 quite brown, with a border of orange spots. I have mentioned 

 this here, because there appears to be good reason for believing 

 that hybridity has a direct tendency to produce hermaphrodibism, 

 though of course it is not the sole cause. 



In the majority of cases, union of the sexes is necessary for the 

 continuance of the species j but parthenogenesis, or the laying of 

 fertile eggs by an unfertilised female, is common in several groups, 

 and is the rule in others. The males of such insects are frequently 

 unknown, and in some cases appear to be actually non-existent. 

 Among the Cynipidce and Apliidce, sexual and seasonal dimorphism 



