HYMENOPTERABEES 153 



Prosopis, feeble in Sphecodes, more clearly visible in Halictus) are represented by 

 prominent transverse rings over the greater part of the worm-like ligula. The hairs 

 on the ligula, which scarcely present any definite arrangement in the lowest stages 

 mentioned, form regular whorls on each transverse ring that can be erected and 

 forwardly depressed. And, lastly, the movable membrane connecting mentum 

 and cardines is extended and supported by chitinous bars, in such a way that 

 when these fold together the mentum is retracted between the stipites as far as 

 the ends of the cardines, while when the chitinous bars unfold it is protruded 

 for their full length. 



Certain modifications of the maxillae are inseparably connected with these 

 specializations of the labium, so that the same successive stages can be recognized 

 in both groups of bees. As soon as the ligula is so far lengthened that it can 

 no longer be retracted into the anterior hollow of the mentum, it is folded up 

 downwards and backwards when not in use, and both in the retracted and protruded 

 condition is sheltered between the laciniae so as to be protected from injury during 

 nest-building and the process of penetrating into nectar-receptacles. The laciniae 

 having come to serve as a sheath for the elongating ligula must therefore extend 

 in the same proportion. The labial palps elongate similarly, in order that they may 

 continue to act as tactile organs, and when the ligula is not too long this is also the 

 case with the maxillary palps. The latter, however, are soon outstripped by the 

 continually elongating laciniae, labial palps, and ligula, and ultimately degenerate, 

 while the laciniae and labial palps keep pace with the ligula even to its highest 

 degree of elongation. The difference between the changes undergone by the 

 maxillary and labial palps, M'hich originally had the same function, is explained 

 as follows. With increasing elongation of the ligula, the laciniae become modified 

 to form a sheath, closely enveloping the ligula, and protecting it from injury, both 

 when retracted and also while being inserted into flower-tubes. During the sucking 

 of nectar the laciniae also assume the role of a suction-tube, in which the nectar 

 probably passes to the mouth by the successive erection of the whorls of hairs on the 

 ligula, from the tip upwards. The labial palps are also pressed into the service of 

 this curious suctorial apparatus, by the flattening of two or three of their basal joints, 

 so as to help the laciniae in the close sheathing of the ligula, the last joint or two, how- 

 ever, retaining the original tactile function. This explains the elongation of the basal 

 joints of the labial palps, which keep pace with the growth of the ligula and laciniae. 

 They become long, thin, chitinous plates enveloping the ligula, while the still tactile 

 end-joints retain their original form and shortness, as well as their free external 

 position. The six-jointed maxillary palps, on the other hand, having been out- 

 stripped by the elongating laciniae, are handed down merely as useless appendages, 

 and therefore present all stages of degeneration from six joints to none at all. 



A final increase in the length of the ligula, beyond that of the structures which 

 ensheathe it, is attained by making the proximal part of this worm-shaped organ 

 (which is fused with the paraglossae) coil twice round when retracted into the hollow 

 end of the mentum. The ligula, therefore, when drawn back, reaches just to the end 

 of its sheath, but when fully protruded projects to a distance about equal to the 

 length of the sheath. The sucking-apparatus of bees, greatly elongated in the way 

 described, is also adapted for boring juicy tissues by the sharpening of the laciniae, 





