That's almost certainly a mere coincidental pairing. The larvae look as if they're moth larvae of some sort, and the large black insect is a Tipulid crane fly. Crane flies are short lived as adults, and many do not feed at all in the adult stage. Furthermore, crane fly larvae are rather diverse - some are terrestrial (known as "leatherjackets", and eaten by insectivorous birds), while others are aquatic, the aquatic larvae being found in fresh water, brackish and even some fully marine habitats. The terrestrial larvae are usually feeders upon various plant roots, and some species can become agricultural pests because of this behaviour. Aquatic larvae will feed upon pond bottom detritus, aquatic plants, and a few have transitioned to become carnivorous, feeding upon
Daphnia and copepods among other organisms. None of the crane flies exhibit parasitism during any part of their life cycles, so those larvae in your photo are perfectly safe.
Your photo also shows in excellent detail, a feature common to all Diptera - the
halteres. These are modifications of the second pair of wings, and in your photo appear as those glass-like strands ending in a blob. These are balancing organs used for orienting the insect in flight. All Diptera have halteres, but these organs are more conspicuous in some flies than others, and are particularly noticeable in those flies whose halteres bear bright colouration. For example, there's a truly enormous (by the usual standards of the Family) Genus of flies living in Panama, the Genus
Pantophthalmus, whose adults resemble super-sized bluebottles, and which have halteres that look as if they've been formed from extruded cornflower blue plastic!
Here's a photo of one of these large flies, and you can see the blue blobs in that photo - these are the halteres.
Meanwhile, back to the crane flies. There are, wait for it,
fifteen thousand species of these worldwide. The UK fauna contains approximately 300 species, but only a minority of these can be identified to species level visually - the remainder are usually dissection jobs. One of the easiest to identify is
Tipula maxima, courtesy of the fact that it's almost twice the size of any other UK species, and has patterned wings to accompany its large size. Your specimen looks superficially as if it could be one of the Phantom Craneflies of the Genus
Ptychoptera, in particular
Ptychoptera albimana, but dissection of the specimen would be needed to be absolutely sure.