NO. I THE INSECT HEAD SNODGRASS 4I 



(1958) that in the sawflies Aglaostigma and Pteronidea "the labrum 

 first appears as a pair of elevations which later become united on the 

 median line." In Pteronidea rihesii Shafiq (1954) says the labral 

 lobes unite at the 28th hour of embryonic life, and the stomodaeum 

 develops immediately behind them. Further evidence of the double 

 origin of the labrum is claimed by Bervoets (1913) to be seen in the 

 individual tracheation of the halves of the labrum observed in an 

 odonate larva. 



It may be conceded, then, that at least in many insects the labrum 

 is formed from paired rudiments, and there is evidence of its similar 

 origin in some other arthropods. The innervation of the insect 

 labrum by nerves from the tritocerebral brain ganglia, which led 

 Henry (1948) to conclude that the labrum is the segment of these 

 ganglia, is interpreted by Butt (1957) as evidence that the paired 

 labral rudiments are the appendages of the tritocerebral segment, 

 which have moved forward to a preoral position and united with each 

 other. Minute tritocerebral appendages have been observed in the 

 embryo of a number of insects, but in most cases they are described 

 as transient vestiges. 



In the Crustacea the premandibular, or "tritocerebral," appendages 

 develop into the large second antennae. Butt suggests, therefore, that 

 it is logical to assume that the crustacean labrum represents the fused 

 basal parts of the second antennae. Yet, in the adult crustacean the 

 second antennae, though they have migrated forward, are usually 

 widely separated from the labrum, and show no evidence of having 

 given up their basal parts to form the labrum, which should have in- 

 volved the loss of their basal muscles. In the lower branchiopods 

 the second antennal nerves are given off from the brain connec- 

 tives near the premandibular ganglia ; in the decapods they arise from 

 the back of the brain. The labral innervation is entirely independent 

 of the second antennal nerves. Finally, in the early crustacean em- 

 bryo (fig. I E) or the nauplius larva a labrum is generally recognized 

 already present before the mouth while the second antennae are still 

 behind the first antennae. In the amphipod Ganimarus, Weygoldt 

 (1958) illustrates the embryonic head region (F) with a well- 

 developed, bilobed labrum overhanging the mouth while the second 

 antennal lobes are yet far behind the mouth. In the Crustacea, then, 

 there is clearly no relation of the labrum to the second antennae. 

 Since the labrum is evidently a homologous structure in all the 

 arthropods, its rudiments in the insects can hardly be identified with 

 the crustacean second antennae, or the appendages of the tritocerebral 

 segment. 



