158 THE SCREW-WORM. 



THE vSCREW-WORM. 



( Campmmyia {Lucilia) macellaria Fab.). 

 This fly is a flesh-flj^ with the additional bad habit of 

 being very impatient, as it does not always wait until the 

 animal is dead that is to furnish the food for the offspring. 

 As a general rule, it lives, like all its relatives, upon decaying 

 animal and vegetable matter in and about which it under- 

 goes all its transformations. The eggs, about one-sixteenth 

 of an inch long, of a light yellow color, are deposited by the 

 female on or near dead animals. The mother must possess 

 wonderful organs of smell, or of some other sense, w^hich en- 

 able her to detect such food, even if well hidden or obscured 

 by other odors. These eggs being quite soft, would soon be 

 destroyed by the direct rays of the sun, hence they are de- 

 posited under the carcass, or on its shady side. Such flies 

 are known to contain as many as 20,000 eggs in their 

 ovaries. These eggs hatch in one or two days, depending 

 upon the length of the time that they were retained in the 

 ovaries by the mother-fly before discovering the proper food 

 for her 3'otmg. The dirty-white larva possesses the usual 

 form of fly-maggots; a ring of bristles between each pair of 

 segments gives the maggot a fancied resemblance to a screw, 

 hence the above popular name. These bristles act as legs in 

 locomotion. The mature maggot is three-fourths of an inch 

 in length, and about one-eighth of an inch in diameter; it 

 possesses a tapering head, that contains two pointed black 

 hooks, which are the mouth-parts used to obtain food. The 

 posterior end of the maggot is very much truncated. The 

 maggots grow very rapidly and reach their full size in four 

 to six days, when they leave the substance upon which they 

 have fed and enter the ground, where, just below the surface, 

 they contract into dark-reddish and barrel-shaped puparia, 

 about one-third of an inch in length. Inside such a puparium 

 the fly is formed, which takes about 7 days, w^hen it leaves 

 both the puparium and the ground to fly about and enjoy 

 life according to its peculiar fashion. This fly (fig. 131, 

 plate XI) is a ver\^ beautiful insect, a little larger than our 

 common house-fly, of a bright metallic green color, and hav- 

 ing prominent dull-red eyes; its back is marked with thfee 



