226 TEXT-BOOK OF ENTOMOLOGY 



Asilus, Xylophaga, and Phora) ; and in the higher Hymenoptera 

 (Crabronidae, Vespidae, and Apidae), as well as in many Lepidoptera 

 (Vanessa, Argynnis, and Pontia), two of the thoracic ganglia are 

 fused together, while all three are partially fused into a single mass 

 in many brachycerous Diptera (Conops, Syrphus, Pangonia, and the 

 Muscidee) ; in certain Hemiptera (Pentatoma, Nepa, and Acanthia) ; 

 also in a beetle (S&rica brunnea). Sometimes the suboesophageal 

 ganglion is fused with the first thoracic, as in Acanthia, Xepa, and 

 Notonecta. The greatest amount of variation is seen in the number 

 of abdominal ganglia, all being fused into a single one or from one 

 to eight. The fusion is usually greatest where the abdomen is short- 

 ened, due to the partial atrophy and modification of the terminal 

 segments which bear the ovipositor, where present, and the genital 

 armature. 



There is only one pair of abdominal ganglia in Gyrinus and in cer- 

 tain flies (Conops, Trypeta, Ortalis, and Phora) ; two in Rhynchaenus, 

 a weevil, and in the flies, Syrphus and Volucella ; three in Crabro 

 and Eucera; four in Sargus, Stratiomys and in butterflies, five in 

 the beetle, Silpha, and in the fly, Sciara, and the moth, Hepialus. 



The nervoiis system in the larvae of the nietabolous orders is not 

 concentrated, though in that of the neuropterous Myrmeleo it has 

 undergone fusion from adaptation to the short compressed form of 

 this insect. 



b. The brain 



The brain of insects appears to be nearly, if not quite, as complex 

 as that of the lower vertebrates. As in the latter, the pair of supra- 

 oesophageal ganglia, or brain, is the principal seat of the senses, the 

 chief organ of the insect's mind. 



It is composed of a larger number of pairs of primitive ganglia 

 than any of the succeeding nerve-centres, and is, structurally, entirely 

 different from and far more complicated than the other ganglia of 

 the nervous system. It possesses a central body in each hemisphere, 

 a " mushroom body," optic lobes and optic ganglia and olfactory lobe, 

 with their connecting and commissural nerve-fibres, and a number 

 of other parts not found in the other ganglia. 



In the succeeding ganglia the lobes are in general motor; the 

 fibres composing the cesophageal commissures, and which arise from 

 the cesophageal commissural lobes, extend not only to the suboe- 

 sophageal ganglion, but pass along through the succeeding ganglia 

 to the last pair of abdominal nerve-centres. 1 Since, then, there is a 



1 This has been shown to be the case by Michels, who states that each commissure 

 is formed of three parallel bundles of elementary nerve-fibres, which pass continn- 



