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TEXT-BOOK OF ENTOMOLOGY 



Fritz Mliller was the first to investigate the mode of development 

 of the wings of the hemimetabolic insects, examining the young 

 nymphs of Termites. He regards the wings as evaginations of the 

 hypodermis, which externally appear as thoracic scale-like projec- 

 tions, into which enter rather late in nymphal life tracheae which 

 correspond to the veins which afterward arise. 



The primitive mode of origin of the wings may, therefore, be best 

 understood by observing the early stages of those insects, such as 

 the Orthoptera and Hemiptera, which have an incomplete metamor- 

 phosis. If the student will examine the nymphs of any locust in 

 their successive stages, he will see that the wings arise as simple 

 expansions downward and backward of the lateral edges of the meso- 

 and metanotum. In the second nymphal stage this change begins 

 to take place, but it does not become marked until the succeeding 

 stage, when the indications of veins begin to appear, and the lobe- 

 like expansion of the notum is plainly enough a rudimentary wing. 



Graber l thus describes the mode 

 of development of the wings in 

 the nymph of the cockroach: 



' ' If one is looking only at the exterior 

 of the process, he will perceive sooner or 

 later on the sides of the meso- and meta- 

 thorax pouch-like sacs, which increase 

 in extent with the dorsal integument and 

 at the same time are more and more 

 separated from the body. These wing- 

 covers either keep the same position as- 

 m the flat-bodied Blattidfe, or in insects 

 with bodies more compressed the first 

 rudiments hang down over the sides of 

 the thorax. As soon as they have ex- 

 ceeded a certain length, these wing-covers 

 are laid over on the back. However, 

 if we study the process of development of 

 the wings with a microscope, by means 

 of sections made obliquely through the 

 FIG. 152. Rudimentary wing of youns thorax, the process appears still more 

 nymph of Blatta, with the five principal" veins simple. The chief force of all evolution 

 developed. . , . . . 



is and remains the power of growth in 



a definite direction. In regard to the skin this growth is possible in insects 

 only in this way ; namely, that the outer layer of cells is increased by the folds 

 which are forced into the superficial chitinous skin. These folds naturally grow 

 from one moult to another in proportion to the multiplication of the cells, and 

 are not smoothed out until after the moulting, when the outer resistance is over- 

 come. 



1 Zur Eiitwickelnngsgeschichte und Reproductionsfiihigkeit der Orthopteren. Von 

 Vitus Graber. Sitzungsberichte d. math.-naturw. Classe der Akad. d. Wissensch.,. 

 Wien. Bd. Iv, Abth. i, 1867; also Die Insekten. 



