22 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



minute branches is conveyed to all parts of the body. The respiratory system 

 of air tubes is found only in insects and their near relatives and differs from that 

 of other animals where the oxygen is carried by the blood. The anterior pro- 

 trusible end of the larva is a very imperfect head, with a mouth leading into a 

 pharynx which is provided with hard, black, curiously shaped jaws. These can 

 readily be seen with the naked eye in the living larva and their action in taking 

 in the soft food should be noted. Each segment of the larva is encircled with 

 a ring of many small pointed teeth used to prevent slipping. The body of the 

 larva is filled mainly by the digestive tract, which is much coiled and folded, and 

 by the yellowish fat bodies, in which the excessive food taken in by the larva is 

 stored up to be used for the next stage in development. In the blowfly larvae, 

 especially when they have become pretty large, there may be seen with the 

 naked eye a sac, the food reservoir, an outgrowth of the esophagus, filled with 

 the reddish liver which has been devoured. Make a drawing of the larva. 



Note the remarkably rapid increase in size, or growth, of the larva. In growth 

 the non-living food is transformed into the living substance of the organism 

 (process of assimilation). What use do the blowfly larvae make of the sand? 

 Which side of the meat do they frequent and why? 



4. The pupae. Record the date on which the motionless brown pupae are 

 first observed. The larvae, when fully grown, cease to feed, become motionless, 

 and shrink, leaving their skins to form the brown pupal cases. Within this case, 

 the adult fly develops from certain rudiments present in the larva, while most 

 of the larval structure disintegrates and serves as food material for the developing 

 adult. As time passes observe the adult structures appearing within the pupal 

 case, the most conspicuous being the eyes and wings. Remove a pupa, examine 

 under the low power and sketch. Is it segmented? In Drosophila the two 

 prominent processes at the anterior end and the less conspicuous ones at the 

 posterior end are the same spiracular openings noticed in the larva, and serve 

 to admit air to the tracheal system. The pupa of the blowfly has no such 

 projections, but the posterior end has a number of short spines of no evident 

 function. 



5. The adults (imagines). Record the date on which adult flies are first 

 noted. Observe that the pupal case is ruptured and left behind by the fly. 

 How many days were required for the entire life-cycle? Would the time be the 

 same for all periods of the year? How many flies were produced? Proportion 

 of males and females? If all the offspring lived to produce at the same rate, 

 how many would there be at the end of three months? What factors in nature 

 prevent this enormous increase in numbers? 



The life-cycle of a fly is a typical insect life-cycle. A sudden transformation 

 in life-cycle, such as that from the larva to the adult, is called a metamorphosis. 

 The transformation of the tadpole (polliwog) into the frog is another example of 

 metamorphosis. On the other hand, many animals have no such sudden meta- 



