NO. I THE INSECT HEAD — SNODGRASS 3 



conveniently, in some cases, on the feet, so that an insect can select 

 its appropriate food without first taking it into its mouth. Insects 

 that have biting jaws masticate their food outside the mouth, and in 

 all insects the duct of the salivary glands discharges extraorally, so 

 that the saliva can mix with the food before ingestion. 



From all this it is evident that a number of advanced ideas have 

 been incorporated in the organization of the insect head that makes 

 it a structure quite different from our own head, and gives the insects 

 advantages that we vertebrates do not possess. 



I. DEVELOPMENT AND EVOLUTION OF THE HEAD 



The insects in their evolution, if we may rely on the embryo for 

 historical information, did not get their modern head all at once. The 

 head of the young embryo, particularly in the more generalized insect 

 orders, is a large lobe at the anterior end of the body, usually itself 

 bilobed (fig. i A, emH), on which are developed the eyes, the an- 

 tennae, and the labrum. Following the embryonic head is the elongate 

 body, which becomes segmented, and eventually on the segments ap- 

 pear the rudiments of paired appendages in the form of small latero- 

 ventral outgrowths. The mouth of the embryo (Mth) is formed 

 ventrally at the base of the cephalic lobe by ingrowth of an ectodermal 

 stomodaeum. In front of the mouth the labrum (Lm) projects usu- 

 ally as a small lobe on the underside of the head. 



The cephalic lobe of the embryo is not limited to the insects ; it is 

 repeated in an early embryonic stage of so many of the arthropods 

 as to suggest that it represents a primary head structure developed 

 by the common ancestors of these animals. This theoretically primi- 

 tive head might be called the archicephalon, but DuPorte (1953) has 

 appropriately named its embryonic representative the hlastocephalon, 

 a term that need have no phylogenetic significance. Some writers 

 have interpreted the embryonic head as representing the prostomium 

 of the annelids, or of the ancestral arthropods ; others contend that 

 it includes primary body segments added to the prostomium. It con- 

 tains the ocular and antennal nerve centers, which become the proto- 

 cerebrum and deutocerebrum of the definitive brain. It is not to be 

 supposed that the size of the embryonic hlastocephalon means that the 

 ancestral arthropods were big-headed animals. The hlastocephalon 

 probably is enlarged to give a precocious start to the development of 

 the contained nerve centers. 



The head of the adult insect includes at least three primarily body 

 segments, and probably some remnant of a fourth segment, which 



