INSECTA. 



jecting and apparently unwieldy structures 

 are rendered exceedingly light, while their 

 solid exterior fits them for all the purposes of 

 strength required by the insect. The large and 

 apparently heavy body of the humble-bee is 

 lightened in a similar manner. In this insect 

 and others of the same order, the vesicles are 

 fewer but very much larger than in Coleoptera. 

 The lateral tracheae in the abdomen form one 

 continuous chain of dilatations, which are 

 larger in the males of the species (Jig- 436) 



Fig. 436. 



The lateral and inferior series of vesicular respi- 

 ratory organs in the abdomen of a male individual of 

 Bombus terrestris. (Newport, Phil. Trans.) 



than in the females. The longitudinal tra- 

 cheae (a), that pass backwards from the 

 thoracic region, are connected just as they 

 pass through the petiole or thoracico-abdo- 

 minal segment into the abdomen, by a very 

 short transverse branch (b), which gives off 

 two pairs of minute branches into the ab- 

 domen. The longitudinal tracheae (c) pur- 

 suing their course onwards are dilated, soon 

 after they enter the large first segment of the 

 abdomen, into two enormously expanded 

 vesicles (/*), above which is placed transversely 

 a third and much larger one, which is formed 

 from the anastomosing branches of the opposite 

 sides of the segment, and is also connected 

 with the little branches given off from the 

 transverse branch (ft). Beneath this large 

 vesicle passes the dorsal vessel, and between 

 the tv\o lateral ones (/") the alimentary canal. 

 Besides the branches from the transverse trachea? 

 (6) there are two others from the large tra- 

 cheae (c), which pass longitudinally backwards, 

 one on each side of the oesophagus. That on 

 the left side (e } e) passes as far as the posterior 



part of the proventriculus, and then turning 

 forwards distributes its branches to that organ. 

 The other on the right (d, d) extends no farther 

 than the anterior part of the proventriculus, 

 immediately behind the crop or honey-bag, 

 upon which it is chiefly distributed. The large 

 vesicle (J") is connected with the dilated 

 tracheae in the succeeding segments, and the 

 whole form one continuous irregularly-shaped 

 vesicular cavity, which, along its under surface 

 in each segment, is dilated into a funnel-shaped 

 transverse trachea (g), that anastomoses with its 

 fellow of the opposite side, passing beneath 

 the muscles as in the larva. From the upper 

 surface of the longitudinal canals similar fun- 

 nel-shaped dilatations (i, k) pass over the 

 dorsal surface of the abdomen and anastomose 

 in like manner with those of the opposite side, 

 besides which single undilated ramifications of 

 tracheae (//) pass inwards on each side and are 

 distributed over the alimentary canal. At the 

 posterior part of the body the vesicular canals 

 communicate directly by a large branch (/), 

 from which large trunks are given to the colon 

 and organs of generation. Thus, then, the 

 use of the vesicles is distinctly indicated, even 

 in the peculiar distribution of undilated tracheae 

 to the whole of the organs of nutrition. The 

 distribution of single ramified tracheae from 

 large vesicles appears to be constant in this 

 order ; it was formerly shown by Leon Du- 

 four* in Scolia hortorum, and we have always 

 found it in the Ichneumonida; and other fami- 

 lies. Burmeister states that he has been un- 

 able to ascertain whether this is also the case in 

 Diptera, in which order the vesicles are both 

 large and numerous. According to Marcel de 

 Serresf the Asilida; have an immense number 

 of small elongated vesicles on each side. In 

 one species they amount to so many as sixty. 

 Burmeister remarks]; that, in Lepidoptera, the 

 vesicles in the Sphingida and moths are chiefly 

 found in the males, which agrees with our own 

 observations in Hymenoptera. In Acherontia 

 Atropos he states also that the existence of 

 spiral fibre in the vesicles is so distinct as not 

 to be doubted. 



This is the structure of the respiratory 

 organs in volant insects, but throughout the 

 class, whether in volant or creeping insects, 

 there is always a complete anastomosis of the 

 tracheae on one side of the body with those on 

 the opposite, as has been well exemplified by 

 almost all insect anatomists, Swammerdam, 

 Lyonet, Marcel de Serres, Dufour, Straus, and 

 others. 



The development of the vesicles begins to 

 take place at about the period when the larva 

 ceases to feed, preparatory to changing into the 

 pupa state. At the time when the larva of the 

 Sphinx enters the earth, and is forming the cell 

 in which it is to undergo its transformation, 

 the longitudinal tracheae of the second, third, 

 fourth, and fifth segments become a little en- 

 larged. In the butterfly, Vanessa urtic&y 

 which does not enter the earth, but suspends 



* Journal dc Physique, Sept. 1830. 



t Mcmoires dcs Mus6um, torn. iv. p. 362. 



j Op. cit. p. 181. 



