INSECTS 313 



as the legs are beloiv it), so that the flying insect is always in a condition 

 of stable equilibrium, exactly as we have seen to be the case in the flying 

 bird (see Part II., p. 142, Section 3. Why, however, are not the wings of 

 birds and of insects homologous structures ?). The wings in the different 

 orders (which see) are constructed in widely different ways, but the 

 following may be said as to their structure in general : They are 

 cutaneous structures, stretched and supported by firmer tubes known as 

 "nervures " (compare with the organs of flight in bats). The firmest of 

 these nervures are found upon the anterior margin of the wings, since 

 this requires to be specially strong for cutting through the air (see also 

 Section 2, c). The nervures are chitinous tubes containing branches 

 of the tracheae (see Section 4) and nerves, as well as blood, which latter 

 must be conveyed to all parts of the body for the purpose of assimilation. 

 The muscles connected with flight (alary muscles) are not situated in the 

 wings themselves, since they would load them too much, but in the 

 thorax, and are attached to the extremities of the wings which project 

 into the thorax (compare with the thoracic muscles of birds). The thorax 

 also contains the muscles which move the legs, and thus, in fact, forms 

 a box of muscles. 



(c) The Flight of Insects. In flying, all insects strike their wings 

 obliquely downwards and backwards, and then raise them obliquely 

 forwards and upwards, so that the tips of the wings describe something 

 like the figure of an 8. In the downward stroke the wings must assume 

 exactly the same position as we have noticed in the bird's wing during 

 the downward stroke (see Part II., p. 150, Section c) ; i.e., the posterior 

 section of the wing, being traversed by fewer nervures than the anterior 

 margin, is impelled by the pressure of air originated by the stroke obliquely 

 upwards, so that the upper surface of the wing is inclined obliquely for- 

 wards and downwards. (This position may be distinctly seen if we push 

 the wing of an insect e.g., a butterfly downwards. Eepeat the experi- 

 ment also with the "artificial wing" described and figured in Part II., 

 p. 151.) Further, the stroke must be followed by the same effect as was 

 noticed in the case of the bird's wing in the downward stroke ; i.e., the 

 insect, in virtue of the force resulting from the resistance of the air, must 

 be lifted up and impelled forwards. If now the wing is once more moved 

 obliquely forwards and upwards, its posterior portion must, in virtue of 

 the air-pressure acting from above, be forced obliquely downwards, so 

 that the upper surface of the wing is inclined obliquely downwards and 

 backwards. The effect of the stroke will be easily understood if we turn 

 the upper illustration in Part II., p. 151 upside down. The insect will 

 in consequence be impelled downwards and forwards. 



If the downward stroke proceeds with greater force than the upward 



