GENERAL PHYSIOLOGY OF THE FROG 21 



1. The flies. Obtain a Drosophila killed by ether and note its character- 

 istic insect structure: the division of the body into head, thorax, and abdomen, 

 the characteristic rings or segments of which the abdomen is formed, the large 

 eyes, three pairs of legs, and single pair of wings (most insects have two pairs of 

 wings, but flies are characterized by one pair). Learn to distinguish male and 

 female fruit flies (A). The males have slender rounded abdomens with a black 

 area in the middle of the tip of the ventral side; the females have broader, more 

 pointed abdomens, and the black markings are at the sides of the ventral surface 

 of the abdomen. In male blowflies, the large eyes nearly meet in front while 

 in the females there is a considerable distance between them. (If the experiment 

 on heredity is to be performed, the two fruit flies given you will differ from each 

 other in some striking way.) 



2. The eggs. Note and record the date on which the eggs are first observed. 

 They are oval white objects, minute in the case of the fruit fly, much larger in 

 the blowfly. Remove one and study under the low power of the microscope. 

 Read carefully the directions regarding the use of the microscope, and consult 

 the assistant on any points that you do not understand. The egg of the fruit 

 fly has hexagonal sculpturings upon the surface which are said to be the impres- 

 sions of the walls of the oviducts, and is provided with two oarlike processes 

 which prevent the egg from sinking into the soft banana pulp, a circumstance 

 which would probably be fatal to the larva when it emerges. The egg of the 

 blow fly is marked similarly but less conspicuously with hexagons, has a concave 

 surface which is the future dorsal side, and a convex surface which is ventral. 

 Draw an egg of either fly. 



3. The larvae. Note and record the date on which the moving, wormlike 

 larvae are first noticed. The larva develops inside the eggshell, and hatches 

 forth rather suddenly by rupture of this shell. Remove a larva to a slide, 

 anaesthetize with ether with the aid of the assistant, cover with a cover glass, 

 and study under the low power of the microscope. Compare its structure with 

 that of the parent fly. Is it more simple? Does it have head, eyes, wings, legs? 

 Is the body divided into regions? Is the segmentation, or ringing of the body, 

 more marked than in the parent? What animals do you know that are similarly 

 ringed along their entire bodies? The significance of these facts may not yet 

 be clear to the student, but they illustrate one of the most fundamental and 

 general laws of development, that every organism in its development passed 

 through stages simpler than itself, and stages that resemble animals lower in 

 the scale of animal life than itself. 



The most striking structure observable in the larva are the tracheal tubes, 

 consisting of a pair of longitudinal trunks running the length of the body, ending 

 posteriorly hi a pair of large openings, anteriorly in a pair of smaller ones, and 

 sending off extensive branches throughout the body. These tubes are full of 

 air which enters them through the openings, called spiracles, and through their 



