66 RECORD OF CURRENT RESEARCHES RELATING TO 



In a very few insects alone are the functions of motion and 

 direction combined. Of these ^shna (Neuroptera) is the type, which 

 flies after the manner of a bird : it owes this faculty to the muscles of 

 flight being attached to the wing itself. It is to be noted, nevertheless, 

 that the abdomen is long and flexible, thereby aiding slightly in the 

 modifications of its course; this is especially visible in the genus 

 Agrion. 



The Lepidoptera ought probably to be ranged in this category ; 

 but the anatomy of their thoracic muscles is not yet sufficiently 

 worked out. 



It is in the Hymenoptera that the first signs of separation between 

 the functions of ti'anslation and direction become apparent. The 

 wings have acquired a strict automatism (tlie axis of suspension 

 remaining fixed), and furnish the motor power only. The abdomen 

 is pedunculated and very mobile. As it is bent or straightened, so the 

 centre of gravity is carried forwards or backwards. In the Cynipidaj 

 and Ichneumonidse this modification is remarkably developed ; if the 

 movement of the abdomen is impeded, they cease to be able to 

 control the direction of flight. 



In the Orthoptera the abdomen has but little mobility, and the 

 function of direction falls to the posterior legs. These being already 

 adapted for leaping are not very efficient in guiding the flight, so that 

 the Locustidte and Acrididae turn very badly. 



In other insects, such as the Coleoptera, the change of direction is 

 effected by the elytra. Raised during flight over the thorax, they 

 form a mobile mass above the centre of gravity, the smallest displace- 

 ment of which influences the position of the latter. The removal of 

 the elytra leaves the motor function intact, but deprives the insect 

 of any power of directing its flight, which is always up or down 

 or horizontal. 



Presenting an intermediate case between the latter and the 

 Diptera are a small group, the Cetonidas, which fly with the elytra 

 down. In this case the elytra act on the axis of suspension. 



In the Diptera the power of direction has arrived at remarkable 

 perfection ; the second pair of wings is transformed into special 

 organs — the halteres — for directing the flight. The halteres act by 

 displacing the axis of suspension, and experiment shows that deprived 

 of these the insect on attempting to fly falls at once ; but if a small 

 weight is attached to the abdomen, so as to bring the centre of 

 gravity behind the axis of suspension, the power of direction is 

 immediately restored. 



Nervous System and Classification of Diptera.* — The well-known 

 variety of development presented by the nervous system of insects is 

 well illustrated by M. Kiiuckel's account of the different arrange- 

 ments in the Diptera, which he finds to mark out their true zoological 

 groups with great distinctness. The principles thus followed were 

 adopted by Cuvier for the animal kingdom, and by Brandt f specially 

 for the group of Hymenoptera. 



* ' Comptes Kendus,' Ixxxix. (1879), p. 491. 

 t See this Journal, ii. (1879), p. 863. 



