Conostigmus, Dahlbom, 1858

Trietsch, Carolyn, Mikó, István, Ezray, Briana & Deans, Andrew R., 2020, A Taxonomic Revision of Nearctic Conostigmus (Hymenoptera: Ceraphronoidea: Megaspilidae), Zootaxa 4792 (1), pp. 1-155 : 147

Distinguishing Male Conostigmus and Dendrocerus ( Table 3)

Dendrocerus and Conostigmus can be incredibly difficult to distinguish ( Mikó et al., 2011). There are both Dendrocerus -like Conostigmus species (including C. lepus and C. triangularis ) and Conostigmus -like Dendrocerus species (such as D. punctipes and D. penmaricus ). It is unclear to which genus C. fasciatipennis belongs. Dessart (1972b) kept the species within the genus Conostigmus based on the presence of the medioventral conjunctiva (medioventral conjunctiva always absent and parossiculi fused with the gonostipes in Dendrocerus ). Dessart (1972b) also noted its cylindrical male flagellomeres, which were typical of Conostigmus .

However, this species also lacks a facial pit (absent in most Dendrocerus ) and has a Dendrocerus -like obtuse ocellar triangle in males (POL greater than LOL), where the two posterior ocelli are closer to the compound eyes than to each other (POL greater than OOL). Molecular data has so far been unable to delimit Conostigmus and Dendrocerus and determine the generic placement for C. fasciatipennis . Based on preliminary ultraconserved element (UCE) data, Conostigmus is paraphyletic and Dendrocerus is polyphyletic, with a few Dendrocerus species ( D. carpenteri and D. conwentziae ) nested within Conostigmus and C. fasciatipennis separating those species from the rest of Dendrocerus (Blaimer et al., in prep.).

Some commonly-used morphological characters for distinguishing Conostigmus and Dendrocerus males are the ocellar ratios and the shape of the ocellar triangle ( Dessart, 1995b; Dessart and Cancemi, 1987). Dendrocerus males are usually distinguished as having an obtuse ocellar triangle (POL greater than LOL) where the two posterior ocelli are closer to the compound eyes than to each other (POL greater than OOL), whereas Conostigmus males are usually thought to have an acute or equilateral ocellar triangle (POL equal to or less than LOL) where the posterior ocelli are closer to each other than to the compound eyes (POL less than OOL).

However, this does not hold true for all Conostigmus . Some species have more obtuse ocellar triangles (POL greater than LOL) in the males, including C. nigrorufus , C. duncani , and C. bipunctatus . Some male Conostigmus even have the posterior ocelli closer to the compound eyes than to each other (POL greater than OOL), including C. lepus and C. triangularis . As of now, there are no known Dendrocerus species with Conostigmus -like ocellar ratios, but more revisionary work on Dendrocerus is needed to investigate this before a conclusion can be made.

Male antennal characters are also commonly used for distinguishing between Dendrocerus and Conostigmus ( Dessart, 1995b; Dessart and Cancemi, 1987). Female antennae are indistinguishable between the two genera, but male antennae of Conostigmus are symmetrical and cylindrical in shape, whereas the male flagellomeres of Dendrocerus are usually asymmetrical and can be serrate or trapezoidal. Some Dendrocerus also have branched flagellomeres ( D. mexicali group), a state which is never found in Conostigmus ( Dessart, 1995a, 1995b, 1999, 2001).

Of course, there are some exceptions in Dendrocerus . Both the D. penmaricus species group and the D. punctipes species group have symmetrical, cylindrical male flagellomeres that resemble those of Conostigmus ( Dessart, 1983b, 1995a). Members of the D. penmaricus group can be differentiated from Conostigmus by the maximum scape width, which is greater than the pedicel length in D. penmaricus species group members (maximum scape width less than pedicel length in all Conostigmus ) ( Dessart, 1995a). The D. punctipes species group is more difficult to distinguish from Conostigmus , but can be differentiated by the combination of the fused parossiculi, absence of the facial pit, and Dendrocerus -like ocellar triangle ( Dessart, 1983b).

Though the characters discussed above can be useful for distinguishing male Conostigmus and Dendrocerus specimens, they should not be used individually to make an identification. We recommend using a combination of characters to distinguish genera due to the numerous exceptions and overlap between these two genera. In addition to the characters above, some useful characters include the presence of the sternaulus (always absent in Dendrocerus , but present or absent in Conostigmus ), wing presence (wings never absent in Dendrocerus , but present or absent in Conostigmus ), notauli posterior end (always adjacent to the transscutal articulation in Conostigmus , but not adjacent in some Dendrocerus ) and male genitalia characters including the presence of the parossiculi (parossiculi fused with the gonostipes in Dendrocerus but never in Conostigmus ) and the medioventral conjunctiva of the gono-style–volsella complex (parossiculi never independent in Dendrocerus , but independent or fused in Conostigmus ). ( Dessart, 1985, 1995a, 1995b, 1999, 2001; Mikó et al., 2011, 2013). The facial pit is present in most Conostigmus (exceptions include C. dimidiatus and C. erythrothorax ) and absent in most Dendrocerus (exceptions include D. carpenteri , D. flavipes and D. rectangularis ). Other characters that can be useful but are more subjective include metapleural sulcus shape (usually curved in Dendrocerus and straight in Conostigmus , though it appears curved in some Dendrocerus -like Conostigmus such as C. lepus and C. triangularis ), as well as head shape (usually triangular in Dendrocerus and globulose or circular in Conostigmus ).