We have already alluded to the fact that, so far as external structure is concerned, there is no great difference between the social and the solitary wasps. Both, too, run through analogous series of forms and colours, and the genus _Ischnogaster_ (Fig. 34) seems to connect the two groups by both its structure and mode of life. The social habits are in many species only inferred, and with greater knowledge will probably prove fallacious as a guide to classification; indeed we have already said that in the genus _Vespa_—perhaps the most perfectly social of all the wasps—there is one species that has no worker, and that lives, it is supposed, as a parasite, in the nests of its congeners. For this species, _V. austriaca_, it has been proposed to create a separate genus, _Pseudovespa_, on account of this peculiarity of habit, although no structural character has been detected that could distinguish it. De Saussure has stated his conviction that workers do not exist in some of the exotic genera, so that it appears highly probable that with the progress of knowledge the present division between social and solitary wasps will prove untenable. Remains of Insects referred to the genera _Polistes_ and _Vespa_ have been found in tertiary strata in various parts of Europe and in North America. [Illustration: Fig. 35—_Masaris vespiformis_. A, male; B, female. Egypt. (After Schaum.)] FAM. 3. MASARIDAE. _Anterior wing with two complete sub-marginal cells. Antennae usually incrassate or clubbed at the extremity. Claws distinctly or obsoletely dentate._ This is a group of fifty or sixty species with but few genera, {89}and most of its components appear to be Insects of the greatest rarity. In their appearance the Insects of this Family differ considerably from the other Diploptera, and as the wings are only imperfectly, or not at all, plicate, it must be admitted that the systematic affinities of the group require reconsideration. The pronotal structure is, however, completely that of Diploptera. The typical form of the Family, _Masaris vespiformis_, though described a hundred years since, is a species of such extreme rarity, and its sexes are so different, that entomologists have only recently been able to agree about it. It has been found in Egypt and Algeria. The genera _Ceramius_, _Jugurthia_, _Quartenia_ and _Coelonites_ are also members of the Mediterranean fauna, while _Paragia_ is Australian, and _Trimeria_ South American. Several species of the genus _Masaris_ inhabit North America, and Cresson has recently described another Masarid genus from the same country, under the name of _Euparagia_.