VI ANTENNATA—THE NERVOUS SYSTEM 467 



IHptcra begins in the families of the Emjiithc, Aslliihc, Therccidcc, Xylophagiihe, 

 Bihionidcc [e.g. Einpis, B), ^Yhel■e the 2 anterior tlioraoic ganglia hecome fused, so 

 that there are only two thoracic ganglia. In this respect the Diptera form a cou- 

 ti'ast to other insects with only 2 thoracic ganglia, e.g. many Colcoptera, Lepidoptf ra, 

 and Hynienoptera, in these cases it is the posterior thoracic ganglion which consists 

 of the 2 fused posterior ganglia. Tabanus (Fig. 325, C) exhibits a nervous system 

 in which all the 3 thoracic ganglia are fused into 1 thoracic ganglionic mass. This 

 is the case in the families of the Syrpliidce, Slratyomidce, and Tahanidcc. The 

 abdominal ganglia show a tendency to approach each other and to fuse. Finally 

 the highest degree of concentration among the Diptera is shown by the Muscidx, 

 (Estridcc, and Pupiparce, where all the ganglia of the ventral chord, except the infra- 

 cesophageal ganglion, are fused into 1 large thoracic ganglionic mass (Fig. 325, D, 

 Snreophaga). From this mass a median nerve then runs towards the end of the 

 abdomen, giving oif nerves to the abdominal segments at regular intervals. 



A series similar to the above occurs in the Coleoptera, but the concentration 

 here rarely goes so far as in the Diptera, since, though the abdominal ganglia may 

 be wanting (in the Lamellieornia), the 2 thoracic ganglionic masses always remain 

 separate. Wherever among insects separate abdominal ganglia are wanting these are 

 fused with the most posterior thoracic ganglion, from which the abdominal nerves 

 then often radiate backward, like the cauda equina in vertebrates. These abdominal 

 nerves, however, may be united on each side into an abdominal longitudinal bundle, 

 or these 2 longitudinal bundles may be fused to form 1 median abdominal strand. 

 The IlJiynchota, the Mallophaga {Oorrodentia), and the Thysanoptera possess a much 

 concentrated nervous system. In many Rhyneliota all the thoracic ganglia, not 

 excluding the infra-cesophageal ganglion, may fuse into 1 ganglionic mass, as is 

 the case in the Coeeidce and also to a lesser degree in the Aphides. 



All other insects have a non-concentrated or else slightly concentrated nervous 

 system, with separate infra-cesophageal ganglion, at least 2 thoracic and several 

 abdominal ganglionic masses, at the most 8 and rarely only 1. 



The full number of abdominal ganglia is not found in any insect larva or imago. 

 In insect-embryos, however, the rudiments of all the 10 abdominal ganglia have been 

 observed. 



It is clear from the above that the arrangement of the nervous system can be as 

 little used as a criterion for the natural division of insects as the structure of any 

 other organic system by itself. It can at the most be used for limiting the sub- 

 divisions within the orders. 



The relation of the larval Nervous System to that of the Imago. — (1) Where 

 the nervous system of the imago is not concentrated, it is generally not concentrated 

 in the larva ; this is evidently the original condition. 



(2) Where single ganglia are fused in the imago, they are often separate in the 

 larva. The honey bee affords an illustration of this ; the bee larva (Fig. 321, p. 463) 

 possesses the fully segmented nervous system : brain, infra-cesophageal ganglion, 3 

 thoracic and 8 abdominal ganglia. The last abdominal ganglion comes from three 

 rudimentary ganglia, which are separate in the embryo. The adult bee (Fig. 320, 

 p. 462) possesses a brain, infra-cesophageal ganglion, 2 thoracic and 4 abdominal 

 ganglia. The posterior and larger thoracic ganglion consists of the 2d and 3d 

 thoracic ganglia fused together ; the composition of the last abdominal ganglion out 

 of 3 ganglia can still be clearly made out. 



(3) Where the nervous system in the imago is much concentrated, it is very often 

 (e.g. Muscidcc) much concentrated in the larva also, and at the same time slightly 

 differentiated. We have here a case of the imaginal characteristics being shifted 

 back on to the larval stage. 



(4) The nervous system in the larva is seldom much concentrated when not con- 



