246 ZOOLOGY FOR MEDICAL STUDENTS chap. 



to form a thread of silk, and in various species including the ordinary 

 silk-worm— the larva of the Moth Bombyx mori — the larva fashions this 

 thread into a cocoon with which it surrounds itself before entering on 

 the pupal stage. 



Of the main orders of insects it is the Diptera that is of the greatest 

 importance to the student of medicine. The most conspicuous charac- 

 teristic is the reduction of the hind-wings to the club-like inconspicuous 

 halteres. In the life-history metamorphosis takes place. The larva may 

 be in the form of a grub or maggot, living amongst decaying plant or 

 animal matter, or leading a parasitic existence. In other cases the larva 

 is aquatic, the details of the more or less elongated body differing in 

 different cases (see e.g. Fig. 107). The many species of Mosquitos^ or 

 gnats are grouped together under the name Culicidae. They feed com- 

 monly on plant juices but in the female, which alone sucks blood, a 

 meal of blood is apparently essential for the complete growth and 

 maturation of the eggs. 



In marshy districts in the tropics, e.g. in tropical America, mosquitos 

 may exist in such numbers as to make life almost intolerable to freshly 

 arrived human beings, although fortunately immunity is eventually 

 developed to the poison of their bite. -The main objectionableness of 

 mosquitos resides, however, not in the irritation caused by their bite ^ 

 but in the fact that they are the transmitters of various disease-producing 

 parasites. In this connexion there are two specially important types to 

 be distinguished, represented respectively by the genus Culex and the 

 genus Anopheles — both of them common in warm and temperate climates. 

 As the anopheline type is responsible for transmitting the parasite of 

 malaria it is important to be able to distinguish it from the culicme type. 

 The correct identification of the species of mosquitos is a matter for 

 specialists but there are certain conspicuous peculiarities which usually 

 enable one to recognize an anopheline mosquito at a glance. The most 

 conspicuous of these is the attitude assumed by the mosquito either 

 when at rest or when about to bite. The culicine mosquito assumes the 

 attitude shown in the right-hand figure at the top of Fig. 107 : the 

 abdomen is nearly parallel to the substratum ; the head and proboscis is 



^ Mosquito is the ordinary Spanish word for a small fly, although in English 

 it has come to be used in a restricted sense for the long-legged gnats called in 

 Spanish Zancudo. 



" The intense irritation caused by the injection of the mosquito's salivary 

 secretion is due not to any poison excreted by the mosquito, but to the 

 presence of symbiotic fungi belonging to the group Entomophthorineae which 

 live in three pocket-like diverticula of the oesophagus and are passed into the 

 wound along with the salivary secretion when the mosquito bites. 



