THE INSECTS: HEXAPODA 85 



in the imago, even though they undergo considerable modi- 

 fication in the process of transformation. In that division of 

 the Diptera represented in our account of the order by the 

 house-flies, flesh-flies, bot-flies, and hover-flies, the change from 

 the larva to imago is so complete that nearly all the larval 

 organs are disintegrated and the organs of the imago are 

 built from separate masses of cells (imaginal disks or buds) 

 which alone escape destruction. The breaking-down of the 

 larval tissues is due to the activity of certain white blood-cells 

 called phagocytes (voracious cells). It has been suggested that 

 the escape of the imaginal disks from the general destruction 

 is due to the fact that these masses of cells alone remain func- 

 tional during the transition period between larva and imago. 



All through the class a division of labor has been reached, 

 in which the young feed and build up the material for the 

 imagoes, which reproduce the species. 



Instinct and Intelligence in Insects. Some of the most inter- 

 esting questions in zoology are those dealing with the various 

 reactions of animals to external objects, the general sub- 

 ject of the behavior of animals. Some actions we explain as 

 due to instinct, others we say are accompanied by intelligence, 

 and still others we believe to be governed by reason. There 

 is the widest possible difference of opinion among naturalists 

 as to the relative importance of each of these in the life of 

 the lower animals. Indeed, there is as yet little uniformity 

 in the definition of the terms themselves, so that in our dis- 

 cussion of this subject in the class of insects we shall follow 

 Professor Lloyd Morgan, of England, in his book, Animal 

 Behavior, by defining as instinctive those acts which are simi- 

 larly performed by all the members of a group of animals ; 

 which are, on their first occurrence, independent of experi- 

 ence ; and which tend, usually, to the well-being of the 

 individual and the preservation of the race. The act of the 

 newly hatched larva of the milkweed-butterfly in devouring 



