904 



INSECTA. 



ful examination and comparison of the parts of 

 the mouth in Volucella,Echinomyia,3ind Musca, 

 with those in Tabanida and Asilidte. To com- 

 mence our observations with the most perfect 

 form of mouth, we find in Tabanidte that the 

 tub rum is an elongated, acute, corneous plate, 

 freely articulated to the margin of the clypeus, 

 and marked along its middle line with a 

 raphe. It is concave on its under surface, and 

 is as long as the mandibles and maxillae, which 

 it partially covers, and somewhat resembles in 

 appearance. In Culex it is longer than these 

 parts, and is more sharp-pointed. In Asilus 

 crabroniformis it is much shorter than either 

 the mandibles or maxillae. It is narrow, tri- 

 angular, and rounded at its apex, with a slight 

 indentation, and is not used by the insect as a 

 lancet, as in the preceding instances, but merely 

 forms the anterior covering of the mouth. In 

 Eristalis Jloreus (e) it is a short mitre-shaped 

 plate, which covers the base of the mandibles 

 and maxillae, and articulates freely with the 

 clypeus. In Volucella bombylans it is reduced 

 to a very narrow short plate, articulated with, 

 and, in a state of rest, inflected beneath a small 

 triangular one, which is inserted into a deep 

 cleft of the clypeus, and which appears to be 

 the proper analogue of the clypeus anterior. 

 In Rhingia rostratait has almost entirely dis- 

 appeared, so likewise has the part that we 

 are inclined to regard as the clypeus anterior, 

 which is inserted into the cleft at the extremity 

 of the elongated rostriform anterior part of the 

 head. In EcMnomyia it still exists as a very 

 narrow corneous plate articulated with the 

 clypeus, inflected beneath the head when the 

 proboscis is retracted, but forming the anterior 



fortion of its base when the organ is extended, 

 n Musca it has entirely disappeared as a dis- 

 tinct piece, but seems to have become the 

 union of two corneous plates, which together 

 form an arch on the front of the mouth, or base 

 of the proboscis, and represent the mandibles, 

 the intervening space being covered by a strong 

 membrane. The mandibles, which had almost 

 disappeared in Lepidoptera, still exist in the 

 rapacious Diptera, and in those which pierce 

 the skin of other animals. In Tabanus they 

 are long, and somewhat lancet-shaped plates, 

 situated immediately beneath the labrum. 

 They are slightly curved, like a cutlass, and 

 sharp-pointed. They are not employed in 

 crushing or cutting solid food, as in proper 

 mandibulated insects, but in puncturing or 

 piercing with a horizontal motion from be- 

 hind forwards, and not from side to side. 

 In this genus, however, their motion appears 

 to be not simply that of thrusting or pier- 

 cing, but also that of cutting vertically with 

 a sweeping stroke, like the lancets of a cup- 

 ping instrument, for which motion they are 

 well adapted by their cotyloid form of arti- 

 culation. In the common gnat, Culex, they 

 are very slender, and sharp-pointed. The pain 

 occasioned by the piercing, or supposed biting 

 of the insect, arises from the act of thrusting 

 these instruments through the skin. In these 

 instances the mandibles are equal in length to 



the other parts of the mouth, but in Eristalis, 

 in which they have still the same acute form, 

 they are somewhat shorter. In the rapacious 

 Asilus crabroniformis the mandibles of the two 

 sides are united to form a single, strong, sharp- 

 pointed barb, very acute, and ciliated at the 

 apex on its upper surface, and projecting be- 

 yond the other parts of the mouth. In Rhingia 

 rostrata they still exist as delicate elongated 

 setae, approximated at their apex ; but in the 

 neighbouring genus, Volucella, we have been 

 unable to detect them, except as two flat plates, 

 approximated to the anterior part of what we 

 regard the proper cardines of the maxillae and 

 labrum, and by which the parts of the mouth 

 are thrust forwards. In Eckinomyia the man- 

 dibles have also disappeared as distinct organs, 

 and seem to be united to the base of the car- 

 dines within the mouth, as in Volucella, and 

 there is a similar condition of these parts in 

 Musca, in each instance the anterior part of the 

 mouth being covered by a strong membrane, 

 which supplies the place of the horny labrum. 

 The lingua exists in most Diptera. It is 

 largely developed in Tabanus, in which it is a 

 single horny seta situated between the man- 

 dibles in the centre of the mouth. It was dis- 

 tinctly pointed out by Savigny, and subse- 

 quently by Latreille. It was called by the 

 former the hypopharynx. The maxilla, like the 

 mandibles, undergo a gradual diminution of 

 size. In Tabanus they are straight, and as long 

 as the mandibles, but narrower and less acute. 

 In Asilus, (jig. 380, g) they are very acute and 



Fig. 380. 



Under surface of the mouth of Asilus crabroniformis. 



m, siib'nentum ; 1, cardo ; 2, stipes j 3, palpifer ; 

 5, lacinia ; h, maxillary palpus ; I, mentuin j 

 i, ligula. 



