246 



COMPAEATIVE ANATOMY. 



are adapted to anotlier mode of ingesting food^ namely, by sucking. 

 In tlie Hymenoptera, in wliicli these organs can function either as 

 sucking or as cutting organs, the parts are of pretty much the same 

 form as in the mandibulate Insecta, but the maxilla3 are consider- 

 ably elongated, as is the labium and its palps. A process, the 

 tongue (lingua), is developed on the surface of the labium turned 

 towards the mouth, and this has two lateral appendages, or secondary 

 tongues (paraglosste), at its base. In many these appendages are as 

 much developed as the tongue itself. 



The mouth parts can be derived from cutting organs even in 

 those insects that are purely suctorial. The Hemiptera and the 

 Diptera have the mandibles and maxillae converted into setas ; in 

 many Diptera the maxillary setae are rudimentary. The labium 

 forms a sheath for these setae, which is firm and jointed in the 

 Hemiptei'a, and soft in most Diptera ; this sheath still carries the 

 labial pTalps, or rudiments of them. There is a rudimentary tongue 

 on the short upper lip (labrum), which is wanting in the Hemiptera. 

 The mouth-organs of the Lepidoptera are differentiated in a 

 different way. In them the maxillae are grooved, and united into a 

 tube, which forms a spirally-coiled proboscis, which is ordinarily 

 of some length ; there are small maxillary palps at its base, which 

 are covered by the palps of the rudimentary 

 labium, which are generally large. 



While gnathites are found on the metameres 

 which fuse to form the head, the succeeding 

 appendages form feet, or locomotor organs for 

 the next three, or thoracic, metameres. The 

 jointing observable in them is of the same cha- 

 racter in all, and shows that they have had a 

 common origin; more marked differences can 

 only be made out in their terminal segments, 

 which are more accessible to adaptation. Certain 

 peculiarities in them are due to their varied 

 modes of adaptation to modified requirements. 



Although the number of three is constant for 

 the feet, in many Insects a larger number can 

 be made out in the embryo; whence we may 

 conclude that they have been derived from forms 

 with a larger number of feet. In the Thysanura 

 the rudiments of these appendages are retained 

 (Fig. 127, p'), even on the abdominal metameres 

 (Campodea). The locomotor processes in many 

 Insect larvEe (Lepidoptera and Tenthredinfe) may 

 be derived from these rudimentary appendages. 

 The paired processes also of the terminal meta- 

 meres in the Thysanura, Pseudoneuroptera, etc. are derived from 

 appendages. 



Kg. 127. Anterior 

 half of the body of 

 Cainpodea fragi- 

 lis. a Antennse. p 

 Feet, p' Eudiments 

 of feet, s Stigma 

 (after J. A. Palmen). 



