Rhizotrogus, which is allied to our May beetle, as also Hydrometra and the Stylopidæ are said to lack the subœsophageal ganglion (Brandt). In numerous Coleoptera (Acilius, Gyrinus, Necrophorus, Melolontha, Bostrichus, Rhynchænus); in many Diptera (Culex, Tipula, Asilus, Xylophaga, and Phora); and in the higher Hymenoptera (Crabronidæ, Vespidæ, and Apidæ), 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 Muscidæ); in certain Hemiptera (Pentatoma, Nepa, and Acanthia); also in a beetle (_Serica brunnea_). Sometimes the subœsophageal ganglion is fused with the first thoracic, as in Acanthia, Nepa, 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 shortened, 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 certain flies (Conops, Trypeta, Ortalis, and Phora); two in Rhynchænus, 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 nervous system in the larvæ of the metabolous 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 supraœsophageal 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.