Tagged discussion on the delimitation and characterization of the tribe Exechiini copied from Tuomikoski 1966:159-164 (Generic taxonomy of the Exechiini (Dipt., Mycetophilidae)):
The subfamily Mycetophilinae, as delimited by Edwards (1925), is a nicely homogeneous and no doubt monophyletic grouping. Edwards divided it into two tribes, as follows: • Anepisternal and pteropleural bristles absent; hind coxa with a fairly strong bristle at base; empodia absent or rudimentary; hind tibial comb usually indefinite or absent; tibial bristles short (Tribe Exechiini) • Anepisternal bristles present; hind coxa usually without basal bristle; empodia and hind tibial comb nearly always distinct (Tribe Mycetophilini)
According: to Edwards, the tribe Exechiini comprises the genera Anatella, Rhymosia, Exechia, Allodia, and Brachypeza; in the tribe Mycetophilini he included the genera: Cordyla, Trichonta, Phronia, Dynatosoma, Mycetophila, Epicypta, Platurocypta, Delopsis, Zygomyia, and Platyprosthiogyne.
Subsequent writers, on the whole, have accepted this division without further comment. However, Freeman (1951, p. 88), when describing a new Mycetophilinae genus from southern South America, suggested that a revision of the tribal limit might be necessary: »Pleurogymnus, gen. n., is interesting because its characters are such as to cast some doubt upon the validity of the two tribes. It is possible that a future reclassification of the subfamily will abolish the tribes and show that some genera such as Trichonta require redefining.» Furthermore, he says (op. c., p. 93): »It is, therefore, probable that the two tribes require a redefinition on characters other than those used by Edwards (1925).»
The words »usually» and »nearly always» in the above cited key of Edwards indicate that the distinguishing characters are not absolutely correlated. In doubtful cases, the author appears to have regarded the absence or presence of anepisternal bristles as decisive. In fact, this is one of the best diagnostic characters, but not absolutely reliable. In many of the Exechiini the mesanepisternum is completely devoid of any kind of macrotrichia, whereas in others at least a part of it is covered with small setulae. In Cordyla some of these macrotrichia are true bristles, not smaller than in the typical Mycetophilini; in consequence the genus was placed by Edwards in the latter tribe, though in other respects it is rather close to some Exechiini, such as Brachypeza. On the other hand, Pleurogymnus FREEM. and Pseudalysiinia TONN. do not belong to the Exechiini in spite of their bare mesanepisternum.
Neither is the presence of a single strong dark basal bristle on the hind coxae an absolute criterion, though it is a useful additional character of the Exechiini. The basal bristle is weak or absent in some Anatella species; many other Exechiini, especially of the compound genera Rymosia and Brachypeza, have two basal bristles (the upper one mostly shorter), and in Cordyla the basal bristle is sometimes replaced by a cluster of a few shorter bristles. A similar, though less conspicuous, patch of tiny bristles also occurs in the Mycetophilini (at least in Mycetophila fungorum); in Trichonta and Dynatosoma the presence of a basal bristle (though usually rather weak and pale in colour) is a common feature.
Nevertheless, the Exechiini are a natural group and can be sharply delimited by other characters. Two of them were not used by Edwards, viz. the absence of a long sagittal line on the head and the shape of the mesothoracic katepistemum. These two features are especially useful, because they seem to be absolutely correlated with each other and with the reduction of the empodium.
Freeman (op. cit.) pointed out that the furrowing of the head along the sagittal suture in Pleurogymnus, in correlation with other characters, places this genus close to some species of Trichonta (Mycetophilini) rather than to the Exechiini, where, in view of the absence of mesanepistemal bristles, it would seem to belong. I have checked this head character for a large number of Mycetophilinae, and found no exception to the rule that the Mycetophilini have a distinct median line extending from the middle ocellus to the highest point of the head, while the Exechiini at most show a short stump of this furrow above the insertion of the antennae at the site of the rudimentary median ocellus. As the presence of a long sagittal line on the head is the rule in other Mycetophilidae (Sciophilinae, of Edwards), its absence in the Exechiini is to be considered an apomorphic character.
In the Exechiini, the lower part of the mesothoracic katepistemum is rounded and somewhat dilated in a characteristic way to cover the base of the middle coxa (see figs. 34–37 in Shaw & Shaw 1951). In the Mycetophilini the lower margin of the katepisternum usually has a distinct anterior angle (figs. 38–41 in Shaw & Shaw) and hardly overlaps the base of the middle coxa; a similar shape of the katepistemum is typical of all other fungus gnats except for the Exechiini which, accordingly, are apomorphic in this respect as well. In one group of Mycetophilini showing a pronounced dorsiventral depression of the whole thorax, the katepisternum is also different, but in another way than in the Exechiini: it is very low and horizontal (figs. 42–43 in Shaw & Shaw) without a distinct anterior angle. The rounded dilation of the lower border of the katepisternum in the Exechiini is probably connected with the peculiar position of the middle legs in repose. Edwards
(1921 b, p. 23) has studied the resting postitions of the Mycetophilinae and found that the Exechiini (Exechia, Rymosia, Allodia, Brachypeza and Cordyla) »all raise their middle legs high above the body, the tarsi being curved towards each other so that they almost meet»
No member of the Mycetophilini is known to have this capacity.
The larval and pupal characters are too insufficiently known to be used in the delimitation of the Exechiini. According to Madwar (1937), the larvae of the two tribes of the Mycetophilinae may readily be told apart by examining the locomotory pads, which are well developed in the Mycetophilini and poorly developed in the Exechiini, though there are exceptions to this rule.
• Sagittal furrow of the head only indicated by a short stump above the insertion of the antennae at the site of the middle ocellus
• Lower part of the mesothoracic katepisternum with an evenly rounded dilation overlapping the base of the middle coxa; middle legs raised above the body in resting position
• Hind coxa usually with a single strong dark basal bristle, sometimes accompanied by a shorter one, or with a small cluster of shorter basal bristles (Cordyla spp.), rarely basally bare (Anatella spp.)
• Tip of hind tibia on the posterior surface usually with a dense field of decumbent setulae; the end bristles below it not forming a pronounced comb
• Empodium rudimentary
• Sagittal furrow of the head long, extending above the site of the middle ocellus close to the highest point of the head
• Lower border of katepisternum with distinct anterior angle, not distinctly covering the base of the middle coxa, or else katepisternum low and horizontal and the thorax dorsiventrally flattened; middle legs not raised in repose
• Tip of hind tibia on the posterior surface without such a dense field of setulae; the end bristles in a more definite comb-like row
• Empodium small but well developed
The revised delimitation of the Exechiini conforms with that of Edwards with the one exception that Cordyla belongs to the Exechiini and not to the Mycetophilini. Of the later described genera Neallodia EDW., as suggested by Edwards, comes near Cordyla, and thus belongs to the Exechiini.
The Exechiini are characterized by some features which, when compared with the situation in the other Mycetophilidae, must be considered apomorphic, thus offering proof of the monophyletic nature of the tribe. Such are at least the reduction of the empodium and of the sagittal furrow of the head, and the shape of the lower part of the katepistemum in connexion with the resting position of the middle legs. This delimitation of the Exechiini leaves the Mycetophilini characterized mainly by the corresponding plesiomorphies and accordingly not easy to establish as monophyletic. In fact, the tribe Mycetophilini may be a paraphyletic grouping, the Exechiini representing only one specialized branch of it. There is rather strong evidence that one group of genera of Mycetophilini, viz. Mycetophila and its allies (Zygomyia, Sceptonia, Epicypta, etc.) with their strong tibial bristles and bristly mesepimeron, constitute a monophyletic group, and that the peculiar Tasmanian genus Pseudalysiinia TONN., with tibial setulae and wing microtrichia not arranged in lines, perhaps represents a »sister group» of all the other known Mycetophilinae (including Exechiini), whereas the remaining genera (Pleurogymnus, Phronia, Zygophronia, Trichonta and Dynatosoma) appear in a certain way to be more related to each other. Thus, in addition to the Exechiini, three tribes might be recognized. However, these questions fall outside the scope of the present study, and for the present there appears to be no need of nor sufficient evidence for a further division of the Mycetophilini.
Introduction to the genera of Exechiini (Tuomikoski 1966:163)
While the delimitation of the Exechiini as a natural and monophyletic group thus presents no serious difficulties, its current division into genera is far from satisfactory, and the problems involved not easy to solve. The present-day generic division of the tribe dates back to Winnertz (1864), whose genera have been accepted practically unaltered by the more modem writers, including DZIEDZICKI, JOHANNSEN, LUNDSTROM, EDWARDS, and LANDROCK. The only exceptions are that Brachycampta WINN. is considered to be a synonym of Allodia WINN., and that one new exotic genus Neallodia EDW. has been added.
Winnertz based the genera nowadays included in the Exechiini mainly on the venation, which is rather monotonous in the whole subfamily, with only comparatively small deviations from the general rule, such as the length of the cubital fork, cu2, and the anal vein. Other characters have only occasionally been utilized.
It is apparent that genera based on single trivial venational features showing poor correlation, if any, with other characters often do not represent natural, still less monophyletic, units. A closer study indicates that this is actually true of some of the genera of Exechiini. Exechia WINN. is probably a partially polyphyletic grouping held together by a single convergent feature, the shortness of the cubital fork. Rymosia WINN., in turn, appears to be paraphyletic, being based on venational characters that are plesiomorphic compared with the situation in Exechia and Allodia. Allodia WINN., again, seems to be paraphyletic insofar as a portion of it perhaps stands closer to one part of Rymosia, whilst another part seems to exhibit a closer relationship to Brachypeza and Cordyla than to the former. Brachypeza WINN. is not quite homogeneous either, since one part of it is different enough to make a close relationship doubtful. Thus of the old Winnertzian genera, only Anatella and Cordyla seem to be properly delimited monophyletic genera.
In the present author's opinion, the current generic system of the Exechiini, however convenient for practical pigeon-holing, cannot meet the demands of a natural taxonomy such as is required, for example, for zoogeographical purposes (see HENNIG 1960). The question is how to master the situation in the present imperfect state of our knowledge.
According to our present knowledge, the Exechiini appear to be mainly of Holarctic distribution and perhaps also origin, and comparatively poorly represented in the tropics and in the Southern Hemisphere, or at least without any greatly deviating or really original types there. It is felt, therefore, that the situation as regards the generic taxonomy of the tribe can be judged in its essential points from a regionally limited material from the main area of the group. The safest step forward seems to be a sharper delimitation and characterization of the smaller units within Exechia, Rymosia, Allodia and Brachypeza as they are seen in the better known European material, a work which was already initiated by the earlier authors, especially Edwards. However, those subdivisions only too often show affinities crossing the customary generic boundaries. This suggests that while more conservative workers may prefer to treat the smaller groups at most as subgenera of the traditional Winnertzian genera, the future aim should perhaps be to group them in a new, more natural way into larger genera irrespective of the customary generic limits. It is not intended to embark on the latter task in the present study. The smaller units are treated as genera in the hope that such a method of dealing with them will at least have the advantage of directing attention to the neglected characters and affinities. In some cases the subgeneric status is employed if the genus in question appears reasonably monophyletic, or simply as a provisional solution in an especially imperfectly clarified case. Of course, a study like the present, being based on but a limited number of the species described, cannot claim to be a final or authoritative contribution to these questions. However, it appears by no means too early to take the more than hundred year old system of the Winnertzian genera under review, and it always facilitates discussion if we provide names for the natural groups which, at least in part, are already familiar to specialists.
A representative collection from Finland, put together in 1962-1965 mainly by Dr. WALTER HACKMAN and the present author, served as the main material for the study. To this was added the other collections in the Entomological Museum of the University of Helsinki, and a fairly comprehensive material from northern Burma, collected by Dr. René Malaise in 1934. During a visit to the British Museum (N. H.) some additional extra-European material was superficially studied.
As to the lists of species included in the genera and subgenera, no completeness was aimed at beyond the Finnish fauna, the study being a preparatory one for an account of the Finnish Mycetophilidae. New combinations are proposed only for species actually studied or more rarely for those which, on grounds of adequate published descriptions and figures, are easy to place.