io6 THE EMBRYOLOGY OF THE HONEY BEE 



ancestors of the highly specialized modern representatives of the 

 arthropods were probably essentially similar to one another, much 

 as they are in the modern chilopods or centipedes. Each of these 

 component segments consists of an annular section of the body 

 wall, provided with a pair of appendages, and containing a pair 

 of ganglia and a pair of coelomic (mesodermal) sacs. In the 

 insects, however, as well as in other modern representatives of 

 the' arthropods, a differentiation has taken place among these 

 originally similar segments, accompanied also by a division of 

 labor. In the insects, for example, a certain number of the anter- 

 ior segments have become united to form the head, the appendages 

 of some of these head segments having meanwhile become altered 

 to form biting or chewing organs ; other segments, such as those 

 of the abdomen, although otherwise but slightly modified, have 

 totally lost their appendages. For the most part the segments 

 composing the insect body are easily recognizable, except in the 

 cephalic region. Here the modification of the primative segments 

 has been especially great, and the identification of these has for 

 the past century been the subject of research, the problem hav- 

 ing been attacked first from the anatomical side, later from the 

 embryological. A complete account of the work done in this field 

 cannot be attempted here, but may be found in such special papers 

 as those of Heymons (i895a), Folsom (1900), Comstock and 

 Kochi (1902), and Riley (1904). 



In insect embryos the segments originally present are much 

 more readily recognized than in the imago, the primative condi- 

 tions being more or less imperfectly reproduced or recapitulated, 

 and our present knowledge of the segmentation of the insect is 

 principally based on embryological rearches. A hypothetical gen- 

 eralized insect embryo, combining the results of investigation on 

 this problem is represented in figure 36. 16 



A primary head and a primary trunk division are recognized 



19 In this figure, borrowed from Snodgrass (1910), one segment of the 

 abdomen is omitted, eleven only being represented. Heymons (1895) has 

 shown that in the Dermaptera and Orthoptera twelve abdominal seg- 

 ments are present. The location of the mouth in the first segment may 

 also be criticized, since it is the opinion of Comstock and Kochi (1902) 

 that the mouth opening is located on the ventral surface of the third 

 (tritocerebral) segment. 



