Brian Jones and I headed to the river on Friday evening. In the back of the car we had our fishing gear but also an assortment of fine mesh nets, trays, magnifying glasses, forceps and droppers because we thought it might be interesting to take a few kick samples to see what kind of bugs there were at the bottom of the river.
In the end the fly rod remained in the car because the invertebrates were quite fascinating and we wanted to take a good look at them. I was very lucky to be guided by Brian who knows them all well. After we had kicked the stones and disturbed the bottom and collected the dislodged critter in a fine-mesh net, we deposited the contents into a plastic tray. Soon the tray became a busy metropolis of miniature forms crawling or scrambling or actively darting around. Modest as they may be in size, these are the bugs make up the critical link between the photosynthetic organisms and the larger more familiar animals like the gypsy barbel that I come to the river to catch as well as the turtles and the wading birds and their predators. The whole ecological edifice is built on on a foundation of photosynthesising algae which can coat the stones or produce fine filaments or drift freely in the water column. Then come the various grazers of algae and their pint-size predators and little sifters and strainers and gobblers of tiny fragments of organic material. These were the kinds of things that were darting or crawling around in our white plastic tray.
Brian knows all of these critters like the back of his hand and needed only a magnifying lens to be able to check the finer points distinguishing one kind from another but it was a real learning experience for me.
Most of what we found were the nymphal forms of familiar insects like mayflies and sedges and midges but there were also snails, tiny limpets, flatworms, segmented worms and leeches in there too. In among this lot were also some tiny gypsy barbel fry which were probably the offspring of fish like those we had seen spawning when Brian and I were last on the river a couple of months ago.
Each of these little creatures is interesting in its own right. I was fascinated to hear, for example, that the leeches we were looking at, lacking the mouthparts to feed insects, prey instead on relatively soft bodied bloodworms. One of these leeches itself was quite red in colour. We had a few bloodworms also in our sample. Their distinctive red colour is provided by haemoglobin in their tissue which helps them to steal oxygen that can be scarce resource in the sediment in which they wriggle around.
In the air above the river we saw loosely swarming mayflies. They were very tiny and we managed to catch one in a net to examine it. Brian told me that these were male Small Dark Olives and that the females would later in the evening be attracted to join them. At the time we came across them it was a little early for the evening courtship rituals and the females were nowhere to be seen. They were off in the bushes somewhere doing their hair and makeup. Evening would be their time to show up and when they did the males would each vie for a position on the edge of the swarm to improve their chances of getting lucky. After their eggs had been fertilised those females would make their way into and then under the water before laying their eggs in dense yellow aggregations on the undersides of stones.
Among the samples we collected the most prominent invertebrates were the caseless caddis (Hydropsychidae), though these were far outnumbered by small mayflies like the angler´s curse (Caenidae). The caseless caddis were the big nymphs that we had seen on our previous visit. These are the size that makes them ideal to imitate and Brian was kind enough to tie me up a half dozen.
Several of the caddis larvae we found had built cases for themselves using small dark stones that they had come across. These were found on the undersides of larger stones. Inside the stone case we were able to see the pupae with antennae curled back waiting for the time when they would make their way to the surface to hatch into the winged adult. I was interested to learn that those caddis nymphs are fishermen just like us. They build a silk net un the undersides of stones which capture organic fragments from the current and funnel them into the “cod end” where the nymph feeds. We were able to see the opening of one such net on the underside of a stone.
Brian tells me that those caseless caddis are not to far away from hatching out and so I must remember to keep an eye open for the adults in a month or so when those slim green riverbed “fishermen” have become fluttering moth-like adults returning to the water to lay their eggs.
Brian tied me up a number of caseless caddis “hydros” and they are a very good copy of the original. One or two were given an extra bit of flash.
fishydreams 
As a keen UK riverfly life enthusiast it was a great experience for me to examine the insect and invertebrate groups living on the riverbed of the Rio Guadalhorce and Rio Grande catchment when I met up recently with Paul in Andalucía. I was surprised first of all to see how much the flow had diminished in the Rio Grande tributary after a couple of months of dry weather. Paul assured me that it would, as usual, dry up completely during the coming weeks although the Rio Guadalhorce, topped up by the dams at El Chorro in the mountains upstream, would continue to flow.
A little research indicated that temporary flows such as the Rio Grande are characterised by recurrent seasonal dry phases of varying duration. It just amazes me how the gypsy barbel and their insect/invertebrate prey can adapt to and survive the major biological and physiochemical changes accompanying such annual droughts and the drying up of their natural habitat.
In general it seems that the diversity of aquatic insects is lower in such extreme temporary waters than in permanent ones. Seasonal succession of species is commonplace in temporary waters, affecting food web structure as certain species temporarily disappear. It was interesting that the many Simulium blackfly larvae that Paul and I had seen here a couple of months ago appeared to be completely absent this time – presumably because they had flown as adults and were now present only as tiny eggs invisible to the naked eye.
It is clear that several insect groups can survive successfully in these temporary running waters. These include certain midges, beetles, crane flies and various bugs, but also amazingly some species of mayfly upwings, sedge/caddis larvae and also dragon fly and damsel fly species – insects that we might envisage to be full-blooded aquatics unable to live outside permanent water bodies.
The most successful insect groups seem to have such adaptations as a highly flexible life-cycle and high powers of dispersal to quickly recolonise newly re-flooded reaches after rain. They often exhibit temperature linked development and adaptations to drought found in the most successful groups include the possession of a diapausing or otherwise protected egg capable of surviving drought and drying out.
Diapause is defined as a period of suspended development in an insect or other organism during regular and recurring unfavourable environmental conditions. It is a physiological state of dormancy and an ecological strategy for the survival of harsh conditions. It may occur during any life stage but is most common among pupae. It is fascinating to think that the sedge pupae we found inside their tough cocoons within their little stone cases may not soon hatch into adults, as I had initially suggested to Paul, but may remain inside and aestivate as the river dries up over coming weeks and become active again and hatch out only when river flows return.
Aestivation is defined as a prolonged torpor or dormancy of an insect, fish or amphibian during a hot or dry period. It is characterised by inactivity and a lowered metabolic rate that is entered in response to high temperatures and arid conditions or drought in warm latitudes. Paul explained to me that gypsy barbel are known to crowd together in the few remaining deep pools as the river dries up and remain largely inactive and can survive providing some water remains.
Insects may also pass unfavourable seasons in the egg stage in a shrivelled state and resume development when moistened. The dried-up eggs of certain midge species are known to hatch as soon as water returns and grow quickly into bite-sized larvae that migrating barbel will surely eat when they recolonise old river haunts as flows recover after rain. Even back in the UK we know that reservoir bank anglers, worried about the drying out of the margins during a previous summer drought, can be surprised the following spring to find trout feeding close in on swarms of midge pupae that have survived – possibly as eggs.
It is indeed reassuring to know that the concept that temporary waters constrain their fauna has been exposed as being based more on human perception than on fact. The gypsy barbel clearly have one-up on us in terms of knowing which prey species can quickly recolonise temporary river flows to provide them with sustenance.
What now puzzles me is how those tiny Rio Grande barbel fry that we found in our kick samples, presumably with limited powers of swimming and dispersal, will survive the near total drying out of their riverine homes as we enjoy the hot summer sun.
Very interesting! Amazing all the activity going on that practically nobody is aware of. As a fellow fly fisherman i could do with honing up up my own entomological knowledge!
I was lucky to have a great guide but it is really so interesting that it´s well worth the effort to find out more!
Hello, I hope you and your family are happy and well. I miss your writing…..Take care….Cody in Texas
Hi Cody. It is lovely to hear from you. Apologies for the delayed response. How are things with you? I hope you are persevering with fly fishing. It is well worth the effort! Best regards to you and your family, P.