1. _longicornis_, Linnæus. 6-7 lines. (Plate VI. fig. 2 ♂♀.) _longicornis_, Kirby.
This genus derives its name from the great length of the antennæ in the male,—_εὖ_, _good_ or _great_, _κέρας_, _horn_. The name of the genus is usually given from some female characteristic, or from a peculiarity common to both sexes, or irrespective of any direct application, but here we find it deduced from a feature exclusively masculine. Instances of the first class we see in _Colletes_, _Halictus_, _Andrena_, _Dasypoda_, _Panurgus_, _Saropoda_, _Ceratina_, _Cœlioxys_, _Chelostoma_, _Heriades_, _Anthocopa_, and _Apathus_; of the second class we have _Prosopis_, _Sphecodes_, _Macropis_, _Anthophora_, _Nomada_, _Melecta_, perhaps _Epeolus_, according to Latreille’s idea, _Stelis_, _Anthidium_, _Osmia_, and _Bombus_; the third class comprises in our series merely _Cilissa_, and in this series the male characteristics that have suggested the name are just as few, being limited to the present genus. But the males among the bees exhibit in many cases strong and striking peculiarities which distinguish them from their partners. Exclusively of the general distinction expressed in their organic difference by the possession of one additional joint to the antennæ and one more segment to the abdomen than is exhibited in the females, we find in many cases in these two parts of their structure very marked singularities. Great sexual differences in the length of the antennæ are not restricted to the present genus; in fact, in most of the genera, this is the first striking feature, but which becomes conspicuously so in some species of _Sphecodes_, in most of the _Halicti_, in some _Nomadæ_, in _Chelostoma_, _Osmia_, _Apathus_, and _Bombus_. In _Eucera_ and _Sphecodes_, each joint of the flagellum is slightly curved, and in the former the surface of those joints appears compounded of hexagons. In _Chelostoma_ the antennæ, besides being longer than in the female, are also very much slighter and slightly compressed, and have a structure capable of curling upon itself; in the female of this genus the organ is clavate; and in _Osmia_, besides their length, in one species the male has a fringe of hair attached to one side along the whole of the organ. In other cases, where the antennæ are not remarkably longer in the male they have extra development by becoming thicker, as in _Melecta_; and in _Megachile_ the terminal joint of their antennæ is laterally dilated and compressed. In scarcely any case are they geniculated at the scape in the male, as they are in the female. The other genera with clavate antennæ have the same structure in both sexes, as in _Panurgus_ and _Ceratina_. Remarkable peculiarities in the terminal ventral segment or segments of the male may be found most conspicuously developed in _Halictus_, _Cœlioxys_, _Anthidium_, _Chelostoma_, _Heriades_, _Osmia_, _Apathus_, Bombus, and Apis. In _Cœlioxys_ and _Anthidium_, and some of the _Osmiæ_, this sex is further furnished with a series of projecting spines, processes, or serrations at the apex of the terminal dorsal segment. In _Chelostoma_, the ventral structure of the male is very singular, the apex being adapted to a mucro at the base which permits the insect to curl up this portion of the body similarly to its antennæ, the furcated extremity of the abdomen fitting, when thus folded, upon the mucro. It is as well to draw observation to these peculiarities, which give additional interest to the study of the group.