122 MANUAL OF ANIMAL BIOLOGY 



life stages, namely, egg, larva, pupa, and adult. This is known 

 as complete metamorphosis, and such Insects are said to be holo- 



METAROLOUS. 



In many other groups of Insects, including the Orthoptera to 

 which order the Grasshopper belongs, there are three life stages, 

 namely, egg, nymph, and adult. This is known as incom- 

 plete metamorphosis, and such Insects are said to be hetero- 

 metarolous. 



Egg. The egg of the Grasshopper is heavily yolked so that an 

 adequate supply of food for the developmental stages is provided. 

 The yolk lies in the center of the egg with a layer of cytoplasm 

 surrounding it. It is this outer layer that divides rapidly fol- 

 lowing fertilization, utilizing the stored food material, and gradu- 

 ally developing to form the highly differentiated Insect. The 

 egg is completely enclosed in two membranes which remain 

 intact until the embryo is ready for independent action, when it 

 breaks through them, or hatches. The inner membrane is known 

 as the vitelline memrrane, and constitutes the true cell wall 

 of the egg. The outer one, the chorion, is a resistant structure 

 secreted by the follicle cells of the ovary which surround the im- 

 mature egg cells. 



Nymph. The immature Grasshopper, or nymph, which hatches 

 from the egg, is quite close to the adult in most of its structural 

 features, but is considerably smaller ; the relative proportions of 

 the various parts are somewhat altered as compared with the adult, 

 the wings are lacking, and the gonads are undeveloped. The 

 nymphs feed and grow rapidly, but, as is the case in all the 

 Arthropods, general increase in size is restricted by the unyielding 

 outer exoskeleton, and so they shed this covering periodically, and 

 form a new one 'a size larger.' Several molts, usually four, occur, 

 and with each one the nymph shows an increase in size and a 

 gradual development to the adult condition. The wings gradually 

 develop from external wing pads, and at the final molt, which 

 ushers in the adult stage, they are fully formed. 



Adult. This is the stage of sexual maturity, and typically in 

 Insects is a flying stage. No further increase in size occurs al- 

 though the adult Grasshopper, as well as the nymph, feed vora- 

 ciously on plant tissues. Mating occurs, and the fertilized eggs are 

 laid in the ground for development as noted above. 



