452 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



The cephalic lobes of the insect embryo do not in themselves show 

 any sign of division into segments, but the presence of three pairs 

 of appendages on them, or closely associated with them, is taken 

 as evidence that the cephalic region includes three primitive segmen- 

 tal areas, and this assumption appears to be confirmed by the fact 

 that the head region of the embryo contains three successive pairs 

 of cavities (coelomic sacs) in the mesoderm. The assumed pro- 

 cephalic segments, or primitive head somites, then, may be distin- 

 guished according to their appendages in insects as the preantennal, 

 the antennal, and the postantennal head segments ; though, according 

 to the names given to the head appendages in Crustacea, they are 

 called the preantennular, antennular, and antennal segments, respec- 

 tively. 



In addition to the segmental areas in the procephalic region of 

 the embryo, there is still the large, anterior apical area on which the 

 labral rudiment is situated. This area, which some embryologists 

 call the acron, would appear to be the equivalent of the prostomium 

 of an annelid worm (fig. 1, Pst)^ since it is preoral in position. The 

 acron has no true appendages, unless the labrum represents a pair 

 of united appendages, but, according to some investigators, the com- 

 pound eyes are developed upon it. Generally it has been supposed 

 that the compound eyes belong to the first true segment, and since 

 the eyes are borne on movable stalks in some Crustacea, it has often 

 been thought that the eye stalks are the appendages of the first 

 head segment. If it is true, however, that there are appendage 

 rudiments on the first segment having nothing to do with the com- 

 pound eyes, the appendage idea of the eye stalks must be discarded. 

 In the light of recent comparative studies on the internal structure 

 of the brain and optic lobes of annelids and arthropods (see Han- 

 strom, 1928), it now appears certain that the compound eyes, the 

 ocelli, and most of the forebrain of the arthropods must be assigned 

 to the prostomium. The preantennal appendage rudiments have at 

 most an evanescent embryonic existence ; the antennal rudiments be- 

 come the antennae of the adult insect, or the first antennae (anten- 

 nules) of Crustacea; the postantennal rudiments are also suppressed 

 in insects, though perhaps they persist in certain species as small 

 lobes of the mature head, but in Crustacea they become the large 

 second antennae of the adult (fig. IC B, 2 Ant). 



If we attempt, now, to translate the facts of the embryonic develop- 

 ment of the head into a phylogenetic concept, we must believe that 

 the ancestors of the insects at some time in their history had a head 

 corresponding with the region of the cephalic lobes in the embryos 

 of modern insects. Since we have no record of earlier stages in 

 the evolution of the arthropod head, though presumably such stages 



