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Physical descriptionThis familiar genus is one of the most extensive in the order; fully 500 species have been described, notwithstanding the fact that owing to their minute size they are usually neglected both by the collector and describer. Although the majority of the species that have been catalogued belong to the European region, yet it is ascertained that the genus is of very wide distribution, and it is probable that it is really as rich in species in tropical regions as it is in Europe. Only three species have been described previously from Mexico or Central America, and I now add upwards of eighty others to this number, though I have left undescribed a large number of specimens that appear to represent different species, only one or two examples of each being present; so that it may be said that evidence has already been obtained of the existence of about 200 species of Apion in our region.
The difficulties in the way of a satisfactory treatment of the genus have been frequently alluded to, but have never been overcome, and no satisfactory way of grouping the species or of sectionizing the genus has been discovered. In order to facilitate the determination of the species here described, I have placed together all those with black legs in one group, and those in which the legs are not entirely black in another; and I have commenced each of these two divisions with the species that have the antennæ inserted quite at the base of the rostrum, placing at the end of each division those that have the antennæ most distant from the base. Although this arrangement is very far from satisfactory, I think it less likely to mislead than any other I can at present devise.
As a rule, the colour of the legs is very constant in each species, but still there are one or two in which it is variable, and also one or two in which it is doubtful whether the legs are entirely black or not; and there are even a small number of cases in which allied species are by this character referred to different divisions.