240 



COLLEGE ZOOLOGY 



and leaves, in rotten wood, humus, and 

 other damp places. The young that hatch 

 from the eggs look like miniature adults. 

 As they grow they molt a number of times, 

 finally reaching sexual maturity and adult 

 functions. 



A grasshopper with 

 gradual metamorphosis 



This is a type of development in which 

 the young are strikingly like the adult in 

 general form of body and in manner of life. 

 However, there is a gradual growth of the 

 body and wings, but these changes take 

 place gradually and are not very great be- 

 tween any successive stages (Fig. 140). 



A dragonfly and mayfly 



with incomplete metamorphosis 



This is a type of metamorphosis in which 

 accessory organs or gills occur in the aquatic 

 naiads, and the adults are aerial. In this type 

 of metamorphosis the changes that take 

 place in the form of the body are greater 

 than in gradual metamorphosis, but much 

 less marked than in complete metamor- 

 phosis. The metamorphosis of a dragonfly 

 (order Odonata) is incomplete. The naiad 

 stage of the dragonflv does not have ex- 

 ternal gills, but has rectal gills and breathes 

 by alternately drawing in and expelling 

 water. 



The metamorphosis of the mayfly is in- 

 complete. Mating takes place during flight, 

 after which the female lays several masses 

 on a stone in the water; each mass contains 

 from 80 to 300 eggs. The eggs hatch in 

 about a month and the young that emerge 

 are called naiads. The naiads live under the 

 water, where they breathe by means of 

 tracheal gills and feed on minute plants 

 (diatoms and algae). Growth is accom- 

 panied by 27 molts and requires from 6 to 9 

 months. When ready to venture into the air, 

 the naiad swims to the surface; a split ap- 

 pears along the back; and a gauzy-winged 



adult flies out to a nearby object where it 

 rests from 18 to 24 hours. Then it molts 

 again and is ready to fly into the air and 

 find a mate. The adult life of both males 

 and females is but a few hours or days. 

 Therefore, the scientific name of the order, 

 which means "living but a day," was well 

 chosen. The mayflies fly to lights in im- 

 mense numbers, in towns along rivers or 

 lakes. One river town has an authentic rec- 

 ord of a pile of dead mayflies 8 feet deep, 

 which formed in one night around an elec- 

 tric light pole. 



A butterfly with 

 complete metamorphosis 



Pieris rapae (order Lepidoptera), the 

 white cabbage butterfly, is one of our com- 

 monest species. The larvae or caterpillars 

 (Fig. 143) feed on the leaves of certain 

 plants, and therefore the eggs must be laid 

 on them. These plants are cabbage, turnip, 

 mustard, horse-radish, radish, etc. The larvae 

 will die if they hatch out on the wrong type 

 of plant. The butterfly probably distin- 

 guishes one plant from another by means of 

 an olfactory sense. The eggs are laid, one by 

 one, few in number, on a single plant. The 

 eggs are bullet-shaped and covered with 

 ridges and depressions. They are fastened to 

 the leaf by the flat end. The larva eats its 

 way out of the distal end of the egg shell and 

 then proceeds to devour the rest of the shell. 

 It begins at once to chew holes in the leaves 

 of the host plant with its jaws, and when it 

 can grow no larger within its cuticular cover- 

 ing, a split appears in the back near the 

 anterior end, and the larva crawls out and 

 expands because of the elasticity of the new 

 exoskeleton. The caterpillar's legs are of 

 two kinds: 3 jointed pairs on the thorax, and 

 5 unjointed, temporary prolegs on the abdo- 



FiGURE 140. Facing page, three types of life cycles 

 found in insects: without metamorphosis, gradual 

 metamorphosis, and incomplete metamorphosis. See 

 Fig. 141 for complete metamorphosis, a fourth type 

 of life cycle. 



