152 MEDICAL ENTOMOLOGY 



The more simple Diptera, such as are found in the Orthorrapha, 

 have as a rule aquatic or semi-aquatic larvae, which move actively about 

 by means of wriggling motions, or with the aid of posteriorly situated 

 fins. The body is segmented, and the head, thorax, and abdomen may 

 be well demarcated from one another. The larva feeds mainly on solid 

 particles, such as animal or vegetable refuse, or on small animalculae, 

 and is provided with mouth parts suitable for this purpose, the 

 mandibles being often well developed. The alimentary tract is as a 

 rule simple, and only slightly exceeds the length of the body, the coils 

 being limited to the hind-gut. Larvae possessing a well defined head, 

 with a pair of mandibles, are termed eucephalous. In the higher 

 Diptera the larvae usually live in a rich food material, such as animal 

 refuse, dung, etc., and neither need not possess biting mouth parts, 

 the mouth being adapted for the ingestion of the rich liquid fluid in 

 which their lives are passed. The same condition has led to a cor- 

 responding degradation of the rest of their structure. The head, for 

 instance, since it has no longer to contain the mouth appendages and 

 their muscles, is reduced until it cannot be recognized as such, when 

 the larva is said to be acephalous-, an intermediate form, in which 

 the head, while yet recognizable, is inconspicuous, is termed hemi- 

 cephalous. The larvae of the majority of Diptera are included under 

 this latter group. Since larvae living in such material do not need 

 to move about actively in search of food, they are generally unprovided 

 with foot appendages, having at most small pseudopodia on the ventral 

 parts of the segments, or thickened annular ridges at the junction of 

 the segments, sufficient to enable them to gain a purchase on the 

 substance in which they lie, while progression is brought about by 

 rhythmical contractions and extensions of the. body. On the other 

 hand, as the outward form has become debased the internal structure 

 has become specialized for the more rapid absorption and digestion 

 of the food. The alimentary canal in these higher forms is al- 

 ways greatly increased in length, as it is in the adult, and is coiled 

 up in the abdomen ; the majority of the coils are situated in the 

 posterior part, which is in consequence generally stouter than the 

 anterior. 



The respiratory system of the larva, owing to its constancy in certain 

 groups, has been singled out as a point of importance in classification. 

 Several forms are met with, dependent on the number and position 

 of the stigmata. These are seldom placed segmentally, but are usually 

 limited to the hind end of the body. From each stigma in this situation 



