182 AN ESSAY ON THE DEVELOPMENT 



there is no difFerence from Andrena or Polisteg, which have heeu ah-eady studied. I 

 have found no sjiecies which shows all the pai'ts more fully developed than Xenoglossa 

 pruinosa (PI. II, Fig. 15). Here all the parts are equally developed and all are func- 

 tional; hence it makes a good starting point. The mentum is not shown in the figure 

 except at the point to w^hich the other parts are attached, and surmounting it cen- 

 trally, we find the ligula ; here a united, though extremely flexible oi-gan. Lying cen- 

 trally upon it, so as to close a groove, is the hypopharynx, in this case not easily separ- 

 able from the ligula. xirising close to the central organ on each side are the para- 

 glossse; almost as long as the glossa itself, flexible, unjointed, flattened and a little 

 incurved at the margins so as to form, when closely applied to it, a partial shield for 

 the ligula. Outside of all, situated at the outer margins of the mentum, are the palpi. 

 These are four-jointed; but the basal joints are enormously elongated in proportion to 

 the terminal two, and they are also flattened out, broadened and infolded, so that when 

 at rest they cover and almost conceal the other labial parts, though not extending for- 

 ward as far as they. In this insect the structures just described are almost entirely 

 covered by the maxillte, and a ti-ansverse section (PI. II, Fig. 15") is interesting and 

 instructive. It represents the structure at about the middle of the combined maxilUe 

 and labium and illustrates the relative position of the parts. 



The tendency in the bees is toward a loss of the paraglossfo, which shorten grad- 

 ually until they disappear altogether, as represented in a species of Bomhus figured in 

 PI. Ill, Fig. 15. Every intergrade is represented in any good series of bee mouth 

 parts, and in their rudimentary condition, without function, they appear in Bomhus sp., 

 re])resented on PI. Ill, Fig. 6. The palpi retain their unique development, and in 

 the figure just cited are seen to be as long as the ligula itself, the basal two joints en- 

 folding it almost completely, while the terminal joints are much reduced in size and 

 set near the tip of the second joint, on the outer side. In other species these terminal 

 joints are proportionately yet more reduced and are sometimes diflticult to find. The 

 essential point to be noted is that at their best development the paraglossse are not 

 jointed and that they tend to complete obsolescence in the most highly specialized 

 types. The palpi in Bomhus require a little further examination : Eeference to the 

 figure last cited will show a short segment between the mentum and the first lon^ 

 joint, and this is membranous in texture. The mouth parts in Bomhus are folded 

 when at rest and the hinge is at the mentum ; hence the necessity for some such i)ro- 

 vision to enable the palpi to bend safely. 



Now let us assume that the ligula of this Bomhus l)ecame rigid and chitinized, 

 and that the edges of the palpi enfolding it became united to form a complete cylinder; 

 and then let us examine Evistalis tenax (PI, lU, Fig. 5) in the light of this assumj)- 



