PLATE II. 



Fig. 12. — Embryo of a Damsel Fly {Calopteryx) . 



(After Brandt.) 



The question of the number of segments in the head of an insect has 

 been much discussed. Savigny (1816) first made the suggestion, which 

 has been accepted by all, that the movable appendages of the head were 

 homodynamous with legs. 



By methods of comparative anatomy, therefore, it has been found that 

 there are at least four segments in the head: i. e., the antennal, the man- 

 dibular, the maxillary, and the second maxillary or labial. The presence 

 of a fifth segment (the ocular) has been suggested, as the compound eyes 

 are borne on movable stalks in certain Crustacea, which might therefore 

 place them among the movable appendages of the head. 



The embryologists have taken up the question and Brandt (fig. 12) 

 has shown in the embryo of the damsel fly that there are distinct seg- 

 ments, each corresponding to a pair of mouth-parts: a, antennal; md, 

 mandibular; mx, maxillary; 2mx, second maxillary. 



The labial and maxillary segments appear to be body segments rather 

 than cephalic. It is thought that perhaps "this represents a phylogenetic 

 stage in which the head proper consisted of fewer segments than it does in 

 existing insects." 



Fig. 13. — Embryo of Hydrophilus. 



(After Heider.) 



Heider found that the suboesophageal ganglion which innervates the 

 mandibles, maxilte and labium is formed by the union of at least three 

 pairs of primitive ganglia. His figure represents a stage in the develop- 

 ment of Hydrophilus, in which these ganglia are still distinct, each pair 

 of ganglia corresponding to a pair of mouth parts. 



Fig. 14. — Head of Embi-yo of Anurida. 



(After Folsciin. I 



Fig. 15. — Section of the Head of an Embryo of Anurida. 



( Af .er Fo!su:ii. ) 



The embryologists having confirmed the conclusions of comparative 

 anatomy, have further demonstrated the existence of vestiges of seg- 

 ments. They have discovered a pair of ganglia between those of the 

 mandibular and maxillary segments. (Fig. 14, si; and fig. 15, 5.) 



Fig. 16. — Head of Embryo of Acilius. 



:. After Patten.) 



As long ago as 1888 Patten figured the suboesophageal ganglion as 

 consisting of four pairs of primary ganglia. 



Some thought that the second of these four pairs of primary ganglia 

 referred to the mandibular ganglia, because they are immediately in 

 front of the maxillary; but according to the results of Folsom (figs. 14 



