NO. I THE INSECT HEAD SNODGRASS 45 



to identify the labrum with the tritocerebral segment or with its ap- 

 pendages, it seems much simpler to accept the labrum for what it 

 appears to be in all the arthropods from trilobites to insects, namely, 

 a preoral lobe of the head. When it is formed by the union of a pair 

 of lobes it practically refutes the idea that it is a head segment, and 

 a forward migration of the tritocerebral appendages that unite before 

 the mouth is hard to visualize as a logical event in evolution. The 

 frequent double origin of the labrum and its dual musculature in 

 insects might suggest that the labrum represents a pair of united 

 appendages ; but the vision of a primitive arthropod having a pair 

 of ventral appendages in front of its mouth is too fanciful to be real. 

 Functionally the labrum is a preoral lip, which may have first served 

 to arrest food at the site of the mouth when pushed forward by the 

 postoral appendages. 



THE EMBRYONIC HEAD LOBE 



The nature of the cephalic lobe of the arthropod embryo, whether 

 or not it is composed of consolidated primitive segments, and if so, 

 of how many segments, has been the subject of endless discussions, 

 arguments and counterarguments, and still the question cannot be 

 considered as definitely answered. Our only source of evidence is 

 the embryo itself. The embryo shows us visible facts, but it does not 

 interpret them in phylogenetic terms, nor does the embryo give us 

 any assurance that it fully recapitulates its ancestral history, which 

 is the very thing we want to know. Hence, whatever phylogenetic 

 interpretations we may deduce from embryogeny are products of 

 our own mental processes, and differ according to our different ways 

 of thinking. Since evolutionary theories cannot be put to an experi- 

 mental test, and we cannot see backward in time, arguments continue 

 because we are ever prone to make the known facts fit a favored 

 theory. 



The principal, externally visible facts about the embryonic head 

 lobe are that it projects anterior to the mouth, shows no clear out- 

 ward sign of segmentation, and bears the first antennae, the labrum, 

 and the eyes when the eyes are developed. A pair of small lobes 

 lying before the antennae, observed in a centipede (Heymons, 1901) 

 and an orthopteroid insect (Wiesmann, 1926), have been regarded 

 as vestiges of preantennal appendages. It is therefore contended that 

 the head lobe includes at least a preantennal segment and an antenna! 

 segment, and some would include an ocular segment. If there is any 

 remote ancestral relation between the arthropods and the annelid 



