130 ENTOMOLOGY FOR MEDICAL OFFICER? 



not end in long spines. The larva is often red or yellow, is stumpy, is 



pointed at both ends, and consists of a head and the very unusual 



number of 13 trunk-segments; it breathes by numerous spiracles 



situated on the sides of some of the segments : in many species there 



projects, on the ventral surface, between the ist and 2nd body-segments 



a curious, often pointed or serrated process, known as the "breast-bone." 



The larvse are sometimes found in decaying wood or fungi, but generally 



are parasitic in living plants, forming excrescences or galls. The 



" Hessian-fly," so destructive to crops in some parts of the world, is the 



larva of a Cecidoviyia. The larvse of a few species bud-off internally, 



during the winter, another generation of larvae ; these consume the 



parent-larva, escape from its empty skin, and may themselves repeat 



and suffer the same process ; and this may go on through several 



generations, until the summer, when the final brood completes its normal 



development. 



Fia. S6.— Gall-gnat : Wing and Antennse. 



Fio. 37.— Fungas-gnat. 



Family MvCETOPHILIDiE : Fungus-gnats (Gr. /uiKijs=fiingus ; 

 0iXfiv = to love). 



In the species of this large family (Fig. 37) the coxa of the legs are 

 usually much elongated and the tibias end in spines ; the antennae are 

 long and slender and have not whorls of hairs ; ocelli, which may be 

 three or only two in number, are generally present; the costal vein 

 stops at the tip of the wing. The larva is worm-like, but has a distinct 

 head with well-formed mouth-parts ; it usually has, like the Cecidomyid 

 larva, numerous lateral spiracles on the body. The larvae are gregarious, 

 and often have a preference for fungi, whence the name fungus-gnats ; 

 some spin a silky web for common protection, some are luminous, and 

 some exude a slimy mucus. Those of certain species of Sciara found on 

 the continent of Europe and in North America, are said to stick together 

 m long coherent strings which progress bodily like snakes— a remarkable 

 instance of puzzling corporate action. The larvje of a minute wingless 

 Mycetophihd are said to damage stored potatoes. A species of Sciara 

 the srth^rn c,?T"l^' '^' Yellow-fever fly, because in certain places in 

 the southern States its appearance in unusual numbers has several times 



