Distinguishing Colaspis louisianae Blake and C. brunnea (F.)
Blake (1974) described C. louisianae based on 29 specimens from south Louisiana, including the holotype, which was collected on soybeans in Paincourtville during 1929. She distinguished it from the much more widely distributed C. brunnea by size, antennal coloration, and relative widths of the dorsal costae. She also reviewed the somewhat hazy origin of the specific epithet and subsequent fixation of the name Colaspis brunnea to the currently recognized North American species. Chapin (1979) clarified the differences between the two species after examining a large number of specimens and noted that most of the external characters are variable enough to make separation of the two species difficult on the basis of Blake's key and diagnosis. Chapin described and illustrated an aedeagal character that will allow unambiguous separation of these two similar species.
Chapin's illustrations include line drawings and scanning electron micrographs of mitten-shaped processes that attach laterally to the base of the internal sac of the aedeagus. The character is diagnostic and Chapin's illustrations are accurate. However, the illustrations do not look exactly like the character in situ when the aedeagus is dissected. Until this character is examined in both species, side by side, and under strong light at high magnification, it is difficult to master. Also, the lateral process is part of the internal sac, which is everted during copulation. Thus, the process may appear in different positions inside the aedeagus, depending on whether the internal sac is completely withdrawn, or partially to fully everted. These latter conditions occasionally obtain in specimens during clearing due to osmotic pressure differentials.
This note addresses the problem of distinguishing these two species by using photographs of cleared aedeagi of the two species. In the two photographs of adults, the obvious difference is size, with C. brunnea (fig. 1) ranging from 3.7-4.2 mm and C. louisianae (fig. 2) ranging from 3.7-5.0 mm. This translates into batches of C. louisianae appearing "a little bigger" than batches of C. brunnea, but smaller members of the two species overlap in size. Thus, size is not useful as a non-ambiguous character. The first costae of the elytra of C. louisianae is wider than in C. brunnea, but again, this is a subtle character that is marginally useful for positive identification.
On fresh specimens, the aedeagal character can be observed by piercing the wall of the abdomen of males and clearing soft tissues away from the median lobe. If museum specimens are desired, the organ can be pulled out the posterior of the beetle using a small gauge insect pin with a hooked tip. It lies laterally inside the body cavity. Clearing is not necessary but makes the character stand out better. The aedeagi of museum specimens can be attached to points adjacent to specimens using a tiny dab of glue or placed in glycerin vials under the specimens. All photographs are based on specimens that had been cleared for a short time in warm 10% KOH or allowed to soak overnight in room temperature KOH.
In the aedeagi of most specimens of C. brunnea (fig. 3), the mitten-shaped processes are not visible, or at least are not obvious, through the wall of the median lobes. It takes the appearance of a dark shadow with poorly defined edges. The structures in the aedeagi of specimens of C. louisianae (fig. 4-5) are always obvious and clearly defined. The reason for the difference is in the relative degree of sclerotization of the margin of the process, and to a lesser degree, its shape. In the aedeagus of C. brunnea (fig. 6), the mitten-shaped process is more elongate and weakly sclerotized distally, and the "thumb" is more elongate and poorly sclerotized. In C. louisianae (fig. 7), the structure is more rounded, with sharply defined edges and consistent sclerotization of the "hand and thumb."
Fig. 1. Colaspis brunnea adult from Missouri.
Fig. 2. Colaspis louisianae adult from Louisiana.
Fig. 3. Colaspis brunnea aedeagus, lateral view.
Fig 4. Colaspis louisianae aedeagus, lateral view.
Fig. 5. Colaspis louisianae aedeagus with interal sac everted, lateral view.
Fig. 6. Colaspis brunnea basal sclerites of internal sac adult, compressed from dorsal slide mount.
Fig. 7. Colaspis louisianae basal sclerites of internal sac adult, compressed from dorsal slide mount. Arrows in all photographs point to the "mitten." Note added during edit: in fig. 3, the arrow should point to the dark area above and to the left of the arrowhead.