Balmy evenings and rising stream temperatures are yielding some sizeable flights of mayflies collectively known as sulfurs. These nuptial displays are most conspicuous as swarms attracted to bright lights near the waterways where the aquatic nymphs spent the past year or more before emerging to take wing for their brief period of adulthood. During the past two nights here at susquehannawildlife.net headquarters, we’ve noted what appear to be three different species of these tiny sulfurs gathering on the walls and doors below entry lamps. Each is referred to by anglers using the common name “Pale Evening Dun”.
Flat-headed mayflies we believe to be Leucrocuta aphrodite, this one a subimago (sub-adult) yet to undergo its final molt, continue their emergence for a third week.Just one of many imago (adult) Leucrocuta aphrodite mayflies seen paying us a visit.Though similar in size to the prior specimens (about one half inch in length), this flat-headed mayfly subimago (sub-adult) may be a different species, possibly Leucrocuta minerva. We first noticed the arrival of these mayflies early last evening.A possible imago (adult) Leucrocuta minerva.Like the preceding mayflies, this sulfur is also referred to as the “Pale Evening Dun”, though you’ll notice it has three tails instead of two and is noticeably larger, approaching one inch in total length. Unlike the flat-headed mayflies (family Heptageniidae) which possess compressed bodies for clinging to rocks and other substrate in the riffles and pools of high-gradient streams, this spiny crawler mayfly (family Ephemerellidae) inhabits lower-gradient waters where it not only clings, but crawls and swims as a means of locomotion. The subimago (sub-adult) seen here is a member of the genus Ephemerella, probably the species E. dorothea. To many fly-tying anglers, it is also known as the “Pale Morning Dun”, the “Little Sulfur Mayfly”, or the “Summer Mayfly”.
Month-by-month and week-by-week changes in water temperatures and other stream parameters trigger the date-specific emergence of flights of particular species of mayflies from the lotic waters where they spend a year or more feeding and growing as substrate-dwelling nymphs. For each individual mayfly, these nuptial flights are only days in duration and include a molt from the sub-adult (subimago or dun) stage to the adult (imago or spinner) stage. Mayflies do not feed during these flights, their sole intention is to mate and reproduce. Conspicuous transitions of large numbers of nymphal mayflies into flying sub-adults are often referred to as “hatches”. So, what’s hatching right now? As local streams reach the upper sixties and seventies, we’ve noticed this mayfly gathering on walls and windows illuminated by man-made light, sometimes a half mile distant from the waterway where they spent the past year or more as a nymph before first taking flight…
A possible subimago (sub-adult) Leucrocuta aphrodite, a flat-headed mayfly sometimes called the “Pale Evening Dun”, a common name applied primarily by fly-fishing anglers to several not-so-closely-related genera of mayflies also collectively known as sulfurs. Flat-headed mayflies (family Heptageniidae) are adapted to nymphal life in stream riffles where their low profile helps them cling to the rocks where they feed.Soon after taking flight, the sub-adult (subimago) Leucrocuta aphrodite molts into a second flying stage, that of the breeding adult (imago), leaving behind its final exuvia. This particular exuvia has a stranded hitchhiker on the abdominal section. Phoretic mites (Acariformes, in part) are thus named because they practice phoresy, attaching to hosts for no other purpose than transportation. Just how small is this little, red-brown mite? Well, the entire mayfly, including tails, is only about one-half-inch long.Freshly molted adult (imago) Leucrocuta aphrodite mayflies spend the day drying out and saturating all their tissues with blood before the search for a mate commences at dusk. Fertilized eggs will be deposited in flowing waters suitable for development of the nymphs.
