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RIVER CRUISERS ON THE POTOMAC
Three species of River Cruisers, Macromia, fly along the Potomac River of the Maryland piedmont. Between late May and September these huge insects with flaming green eyes are observed in stately flight over the river, but rarely do they land for a close look. Their flight is fast, but very steady, and it is SOMETIMES possible to identify them in flight by noting the details of the yellow pattern on shoulder and abdomen. This is tricky, and most identifications are best left as probable. The photos below give some pointers. They usually rest high in trees, but they also like to feed over cornfields near the river, and sometimes hang perched under the leaves, offering a good view.
Royal River Cruiser Macromia taeniolata. The largest species, up to 4 inches long. Males appear more slender than males of the other two species, with a narrowed (rather than slightly flared) tip to the abdomen. (Females of all three have thicker abdomens.) The yellow spots on the abdomen are smaller, with the spot on segment seven slightly larger. Has two yellow shoulder stripes (hard to see). Found on the Potomac in DC and Montgomery Co. Also on a few other larger rivers on the piedmont and coastal plain, where not brackish. It may visit, or colonize, large lakes, such as Little Seneca Reservoir. As of 2002 there was a healthy population at Clopper Lake, Montgomery Co. MD.
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Macromia
taeniolata, Royal
River Cruiser
Macromia taeniolata, shed larval skin
llinois
and Royal in flight
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Royal
River Cruiser males in flight
Illinois (Swift) River Cruiser Macromia i. illinoiensis. The commonest and most widespread of the three. Found through out the state on medium and large streams and rivers. Usually told from Royal by smaller size, and slightly thicker tip to abdomen. The yellow spots on the middle segments of the abdomen are reduced or lacking, and most of the abdomen looks black, with a very large yellow spot on segment seven. There are no yellow shoulder stripes. The above applies to the piedmont. On the coastal plain is the closely related subspecies Macromia i. georgina, with more yellow on the abdomen, and yellow shoulder stripes. The two forms appear to intergrade along the fall line.
Allegheny
and Illinois
River Cruiser females, comparison
Allegheny
and Illinois
River Cruiser males, comparison
Illinois in flight
teneral
illinoiensis, wing damaged on emergence
Allegheny River Cruiser Macromia allegheniensis. The least common, least known of the three. Size near that of Illinois, but males often appear slightly more slender. Has yellow spots on middle segments of the abdomen, unlike illinoiensis, and a much larger spot on segment seven, unlike Royal. Very close to georgina, but shoulder stripes lacking or vestigial. Positively identified only in the hand. Probably usually separated from illinoiensis by having the lateral yellow spot on segment two not divided. Known from the Potomac and side tributaries from Montgomery to Allegany Counties. May not overlap with georgina. Where it occurs with illinoiensis, it seems to prefer smaller streams, but it is found on the largest stretches of the Potomac as well.
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