NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS 25 



ture cannot be fully described here ; the student must refer to special 

 papers on the subject or to more general texts for wider information. 

 In the study of any insect head, however, an attempt must always 

 be made to homologize the special features encountered with the 

 fundamental head structure from which the specialized types pre- 

 sumably have been derived. To correlate structural evolution with 

 changes in function is the essence of morphology. 



To understand fully the nature of the insect head it would be neces- 

 sary to know its phylogenetic evolution. This we cannot know, but we 

 can infer something about it from embryonic development. The pri- 

 mary embryonic head in all the arthropod groups is a large lobe at 

 the anterior end of the body on which the eyes and antennae are 

 developed, and which contains the primitive brain ganglia. This 

 protocephalon or first head, therefore, is entirely a sensory region. 

 The mouth is formed by ingrowth of the stomodaeum at the base 

 of its under surface. If the protocephalon truly represents the primi- 

 tive head of arthropods it might well be termed the archicephalon. 

 But as the head of the embryo, without any phylogenetic implica- 

 tions, it has been well named the blast ocephalon by DuPorte 

 (G. blastos, a bud or sprout, generally in embryology for the first 

 recognizable beginnings of something, as in blastoderm, blastopore, 

 ectoblast, etc.). 



Behind the protocephalon of the early embryo is a region of four 

 body segments in front of the thorax. The first of these segments 

 in some insects bears a pair of minute, transient limb vestiges which 

 correspond to the second antennae of Crustacea, the second is the 

 segment bearing the mandibles, the third is the segment of the first 

 maxillae, and the fourth is that of the second maxillae which unite 

 with each other in insects to form the labium. These four segments 

 are eventually consolidated with the protocephalon to form the defin- 

 itive head. The ganglia of the first of these segments are drawn 

 forward and unite with the protocephalic brain to become the trito- 

 cerebral lobes of the definitive brain. The ganglia of the other three 

 segments combine to become the suboesophageal ganglion of the 

 mature head. These are the visible facts of the embryonic develop- 

 ment of the head. Theories on head segmentation are not so simple. 



Inasmuch as the embryonic head lobe lies in front of the mouth, 

 bears the antennae and the eyes, and contains the primitive brain, 

 it has been interpreted as representing the prostomium of the annelid 

 worms (Holmgren and Hanstrom). This idea gives a very simple 

 concept of the relation of the arthropods to the annelids. More 

 recent embryological studies on the arthropods have, however, re- 



