The Large Red Damsel is one of four European species whose bodies are predominantly red. Compared to its smaller stable mate the Small Red Damsel, P. nymphula is a relatively large and robust damselfly. It's widely distributed and generally quite common throughout Europe but, it tends to be much more localized in the South. The species' main distinguishing features include, obviously, its red-and-black thorax and abdomen (although some female colour forms combine black and yellow), its black legs, and its black pterostigma.

Several forms have been distinguished based on female patterning, and all have yellow bands around the abdominal segments, but variation in this species is poorly-known. The typical form, includes populations known as typica and intermedia; others are characterised by females with less (form fulvipes) or more (form melanotum) black dorsal patterning. fulvipes form females also exhibit a black hind margin to the pronotum, while melanotum females have yellow (never red in mature specimens) antehumeral stripes and may altogether lack red colouration on most of the abdomen.

Throughout most of Europe, the large red damselfly's colour and early emergence distinguish it from most co-occurring species. Small red damselflies (Ceriagrion species) have reddish-brown legs and pterostigma (the pterostigma is greyish in the large red damselfly). Dragonflies (e.g. Sympetrum species) are more robust, have a characteristic wings-splayed resting posture, hindwings larger than forewings, and eyes that make contact over the top of the head (as opposed to the broadly separated eyes of damselflies). Large red damselflies potentially co-occur with the poorly-known Greek red damsel (P. elisabethae) in parts of the Balkans. This species cannot be distinguished in flight, but captured males can be separated by the dissimilar anal appendages. In P. nymphula, the upper and lower appendages are of equal length, with a hook below the upper appendages extending almost to their tip. In P. elisabethae, the lower appendages are always slightly longer than the upper, and the hook is retracted almost to the base of the upper appendages. Female Greek red damselflies have two prominent ridges to the rear of the pronotum, visible in profile and dorsal view; these are much reduced in large red damselfly females.
 
 

Preferred environment is streams, small pools and lakes; generally prefers acid sites often with little or no open water. Flight period from mid April to end of September according to locality. 

Adult males are non-territorial, but can behave aggressively towards other damselflies, behaviour which includes attempts to physically force an opponent into the water or ground. Most aggression is directed towards members of the same species, but small red damselflies (which have a similar appearance) may also be victims. P. nymphula is known to eat a wide variety of prey items in the larval stages. Prey items include crustaceans as small as 0.8 mm, and animals prey on progressively larger organisms as they develop, including worms, larger crustaceans and a range of insect larvae, although predation on members of the same species is rare. 

The large red damselfly is highly efficient at extracting energy from its diet, absorbing almost 90% of available energy from food items. Although dragonflies are primarily visual animals, P. nymphula has been found to respond to chemical traces left by predators, foraging less when chemical signals of a predatory dragonfly larva is detected.
 

The large red damselfly is semivoltine, taking two years to complete larval development, developing to a late larval stage in its first year and overwintering prior to emergence. This long developmental period is associated with a low rate of larval survival; as few as 0.2% of larvae may complete development in fish-free ponds (Lawton, 1970). Maturation takes 9-15 days to complete, and adults may live for a further 40 days or more (with males exhibiting greater average lifespans than females).

Eggs hatch 2-3 weeks after laying. Development takes 2 years, with final instar larvae in diapause over second winter. Larvae are bottom dwellers amongst plant debris and vegetation. Diet consists of insect larvae, protozoa, rotifers and micro-crustaceans, with later instars taking more insect larvae. Larvae are territorial.

Larva is short and rather squat in comparison with 'typical' small damselfly larvae. This appearance is due to extended wing sheaths, which reach almost to abdominal section 6. The head is squarish, with a straight line forming its rear margin. Two dark bands are present on the femur. Larvae have broad, strongly pointed lamellae with characteristic dark markings, often taking the form of an 'X'. Colouration is dark, typically brownish, although this may result from staining  In contrast to many species, large red damselfly larvae can typically be identified in all stages of development, not simply the final instar.

Larvae are 'cryptic claspers', concealing themselves from predators by remaining fixed to debris and vegetation that acts as camouflage, a strategy which appears to defend them effectively against fish. They may also 'play dead' when disturbed in an effort to avoid predation. 

Larvae are ambush predators and appear to adopt a conservative feeding strategy, expending little energy in pursuit of prey but limiting their potential food intake. They exhibit a range of territorial behaviours when faced with conspecifics, including jabbing the caudal lamellae at competitors and engaging in 'staring contests' until one animal withdraws.

Emergence occurs from as early as the beginning of April to mid-August, occasionally to late August, with a peak between May and June. Most emergences take place in early- and mid-May, and animals emerge low down on bankside and emergent vegetation. Emergence is often synchronised so that large numbers of new adults take to the wing at once. Immature adults mature away from breeding sites for 2 weeks. Males defend perch near breeding site against rival males and attempting to gain females. Mating occurs in vegetation.