iQoS] Habits of Insects as a Factor in Classification 75 



THE SEDENTARY HABIT. 



Insects in many different groups show a 'tendency to become 

 fixed for part or all of the life cycle, and in some cases they reach 

 very extreme conditions in this direction. It is most pronounced 

 in the group of Hemiptera, and in this group it shows a very com- 

 plete series of gradations from the free forms to those which are 

 most extremely sedentary. In the Psyllidse for example there 

 are some forms which show a tendency toward attachment dur- 

 ing the larval stages, but in others this is modified into the gall 

 infesting habit. In the white flies there is fixation during the 

 larval and pupal stages with the free adult. In Aphids there 

 are many species which attach themselves by the mouth parts 

 and remain practically fixed for long periods. This is notably 

 true of such species as the beechblight, alderblight, wooly aphis 

 and others. Among the scale insects we have the most extreme 

 condition, but there are gradations from such free forms as the 

 mealy bugs and the Orthezias through the unarmored scales to 

 such very extreme forms as the Diaspids. There can be no ques- 

 tion, it seems to me, that these series show the lines of divergence 

 and that the more extreme attached species must have descended 

 from those which had greater freedom, and that the sedentary 

 habit has in this group become more and more fixed. 



Some of the moths show a considerable tendency to fixation, 

 and examples such as the case bearers and bag worms and the 

 tussock moths show steps in the direction of fixation for certain 

 portions of their existence. 



COMMUNITY LIFE. 



The community life in insects is another very striking line 

 of adaptation and one in which Vve can see most positively the 

 gradual derivation from the more primitive forms in which com- 

 munal life is unknown. In its simplest condition it may be con- 

 sidered as represented in the aggregation of larvgs which remain 

 and feed together for a considerable period, but in which there is 

 nothing more than a mutual protection, construction of tents or 

 other devices, and no development of distinct kinds of individuals 

 as a result of social habit. We have, however, amongst the ter- 

 mites, the ants and the bees, extreme cases where the community 

 habit resulted in a very striking production of different kinds of 

 individuals in a colony, these being adapted to carrying on entirely 

 different activities. While termites, ants and bees must be con- 



