150 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



the proboscis appear to depend on the greater or less develop- 

 ment of the several parts which can be recognised in the 

 immature Blow-fly, with the presence or absence of paired 

 lancets, which do not appear at any stage of development 

 in the Muscidse. 



The position of the palpi, which are always present, has been 

 too much neglected in the comparative study of the mouth 

 organs of the Diptera just as the position of the antennae has, 

 in the study of the head capsule. In the Tipulidse the palpi 

 are near the extremity of the proboscis, and the greater part of 

 the organ corresponds with the rostrum, whilst in the Taba- 

 nidse and the Gnats (Culicidse) they arise close to its base, the 

 rostrum being exceedingly small or entirely absent. The parts 

 developed from the discophore may be very large, whilst the 

 theca is rudimentary ; or the discophore may, as in the flies, 

 become a portion of the dorsum of the oral sucker, so that it 

 is apparently absent. 



The Maxillary Palpi are usually three or four jointed. In the 

 Muscidas, however, there is never more than a single joint. 

 These palpi, in the Diptera with paired lancets, usually form 

 a sheath for them, as in the Tabanidse. 



The Rostrum is very generally highly chitinized, and forms a 

 kind of beak covering a fulcrum very similar to that of the fly. 

 In such cases the proboscis is incapable of being withdrawn into 

 the head capsule sometimes, as in Bombylius, the rostrum is 

 very short, and the fulcrum is then rudimentary. 



The Prelabrum is always a prolongation of the dorsal portion of 

 the rostrum, with which it articulates; but it is usually free, and 

 does not articulate with the ligula, so that it can be lifted from 

 the groove in the pseudolabium. 



The Pseudolabium can generally be folded back as far as the 

 base of the palpigerous scales to which it is always attached. 



In the Crane-flies (Tipulidcs), however, it is very short and 

 encloses the labrum, as in the true flies. In Bombylius it is of 

 extraordinary length. The oral sucker varies greatly in form, but 

 in the Crane-flies it so closely resembles a galea formed of two 

 joints that it is difficult to understand why it has so long been 



