12 MEDICAL ENTOMOLOGY 



the integument at a small pit. At the base there is a small bipolar gang- 

 lion cell. 



In the Muscoidean flies the integument is adorned with many large 

 bristles, called macrochaetae. These are arranged in a definite man- 

 ner, providing most useful characters in classification. Since their 

 value was pointed out by Osten Sacken great attention has been paid to 

 them by dipterologists. A discussion of chaetotaxy, as this branch of 

 descriptive entomology is termed, will be deferred until the regions on 

 which they occur have been described. 



Osten Sacken has some interesting remarks on the possible function of 

 these large bristles. It had been previously suggested by Macquart that 

 their purpose was to protect the fly from collisions, such as its rapid flight 

 might render it liable to, and the ingenious author carries this suggestion 

 a little further. He points out that macrochaetae occur almost solely in 

 those flies which are pedestrian in habit, using their legs in the search 

 for food, in attacking other insects, and in oviposition, while in those 

 flies which are mainly aerial in habit, and use their legs only for alighting, 

 large bristles are rare. In Tabanus, for instance, which merely uses its 

 legs so far as is necessary in finding a suitable place to make a wound in 

 the skin of the host, the bristles are insignificant, while in the domestic 

 house flies, which run about over the food surface, and search place 

 after place, the bristles are well developed. In the Tachinidae, which 

 parasitise caterpillars, many large macrochaetae are always present. 

 It is evident that flies of an aerial habit, which spend their lives mainly 

 either at rest or on the wing, and which have moreover the faculty of 

 poising or hovering, will be much less liable to sudden collisions than 

 those which have a pedestrian habit. He concludes that the macroch- 

 aetae are organs of orientation, fulfilling much the same function as the 

 whiskers of a cat. 



The exo-skeleton is not of uniform thickness throughout, but is 

 divided up into numerous plates, each of which has received a distinct- 

 ive name ; these are often separated from one another 

 The nomenclature u , . i ,, ,. . . . . 



of the exo-skeleton y membraneous intervals. The divisions are mainly 



in the transverse direction, and represent typically the 

 lines of separation of the metameres ; in the Diptera, they are, of 

 course, greatly modified from the primitive condition, and it is often a 

 matter of some difficulty to decide which of the primitive plates a parti- 

 cular portion of the integument may represent. The subject is of 

 academic rather than of practical importance, and it will suffice if it is 

 touched on it only so far as to explain the terms in use. 



