12 ANArOAfV AND MORPHOLOGY OF INSECTS. 



system, consisting of branching tubes called tracheae. These 

 usually open externally by a series of lateral spiracles, or 

 breathing pores, on or between the obvious segments of the 

 thorax and abdomen. Even when spiracles are absent and the 

 tracheae exhibit no external opening, these tubes are always 

 found filled with air. 



Insects have been defined as arthropods, or animals with 

 jointed limbs, with a distinct head, thorax and abdomen, and a 

 respiratory system consisting of tracheal vessels. 



Morphology, or the relation of the various parts of the body 

 to each other and to the corresponding parts of other animals, 

 forms the basis of all scientific anatomical nomenclature. 

 Without a knowledge of these relations all nomenclature 

 would be uncertain and arbitrary. Without such knowledge 

 each author would give an organ or part a name, one name 

 would be as good as another, and inextricable confusion would 

 result. All sound morphology depends upon a study of the 

 developmental process. 



In insects development is often discontinuous, progresses 

 rapidly in the ^g^ to a certain stage, and then suddenly ceases, 

 or appears to retrogress. After a longer or shorter period 

 of growth it recommences, and gives rise to remarkable 

 metamorphoses. Development in the egg is spoken of as 

 embryology; that of metamorphosis, as after -development 

 (nach-embryologie). 



Embryology. — The ovum of an insect consists of a germ and 

 a vitellus, or food -yelk. The relation of the germ to the 

 vitellus has long been a problem awaiting solution.* 



The germ consists of a germinal vesicle and a germinal spot. 

 It corresponds to the cicatricula of the bird's egg ; the vitellus 

 also corresponds to the yelk of the egg in birds and reptiles. 

 The egg is enclosed in two membranes — an external shell or 

 chorion, and a thin internal membrane, the yelk sac or vitelline 

 membrane. These, like the shell and vitelline membrane of 

 the bird's Qg'g, take no part in the formation of the embryo ; 

 their function is merely protective. 



* Balfour, 'Comparative Embryology,' 1881. 



