STRUCTURE, GROWTH AND ECONOMICS OF INSECTS 



33 



the blood system, muscles, reproductive organs, fat-body, etc. The 

 division of the mesoderm into two layers produces paired cavities in 

 each segment — the ccelomic pouches (Fig. 37). 



{b) Metamorphosis. — The various changes that occur after the 

 hatching of the egg are comprised under 

 the term metamorphosis. 



After the escape of the embryo from 

 the egg the development may be (i) holo- 

 metabolic, i.e., with complete metamorpho- 

 sis, the insect passing through two distinct 

 phases, larva and pupa, before assuming 

 the adult form; (2) heterometaboUc, i.e., with 

 incomplete metamorphosis, without a dis- 

 tinct pupal stage, the larva being like the 

 adult but without wings or mature repro- 

 ductive organs; (3) ametabolic, i.e., without 

 metamorphosis, the young being Hke the 

 adult. Insects belonging to the orders 

 Neuroptera, Mecoptera, Trichoptera, 

 Lepidoptera, Coleoptera, Diptera, Sipho- 

 naptera, and Hymenoptera are holome- 

 tabolic; the Orthoptera, Platyptera, Ple- 

 coptera, Odonata, Ephemerida, Thysan- 

 optera, Homoptera and Hemiptera are 

 heterometabolic; and the Thysanurans 

 and Collembolans are ametabohc. Most 

 insects are oviparous, but some like cer- 

 tain scale insects and Sarcophagidae are 

 larviparous. 



Larval Stage. — In general two types 

 of larvae are recognized : the thysanuriform 

 and the eruciform. The former type is 

 considered quite generalized and primitive 

 in form, and is common among the hetero- 

 metabola. The body is flattened, the legs and antennae are long, 

 the caudal cerci are well-developed, and the mouth-parts are 

 mandibulate. 



Pig. 37. — Embryo of jEcan- 

 thus, ventral aspect, a, An- 

 tenna; a.^-a.^, abdominal ap- 

 pendages; e., end of abdomen; 

 I., labrum; li., left fundament 

 of labium; Ip., labial palpus; 

 L^-l.^, thoraciclegs; w., mandi- 

 ble; tnp., maxillary palpus; mx., 

 maxilla; p., procephalic lobe; 

 pr., proctodoeum. {From Fol- 

 som after Ayers.) 



