Identification

Is it a Dolomedes?

As adults, the two European Dolomedes species are usually striking and distinctive in appearance. As well as being very large as adults (females ca 18-23 mm and males ca 13-18mm body length), they are characterised by a white, cream or yellow stripe along the sides of both the carapace and abdomen which contrasts with the dark body. However, unbanded adults also occur, particularly in D. plantarius populations (Fig. 1c & 4b), and can be more difficult to distinguish from other spider species, as can both banded and unbanded juveniles. At all growth stages there is considerable variation in body colour and well as the colour and width of the stripe. Body and stripe colour may be the same or differ between the carapace and abdomen. Stripe width and boldness is also very variable and some individuals have paired white spots along the abdomen. 

The size range, and aspects of the patterning and colour variation all overlap with those of other European spider species but the overall appearance of the European Dolomedes species remains distinctive at all growth stages. Their bodies are essentially cigar-shaped and powerful in appearance, tapering towards both ends. The legs and body are shortly hairy: when the spiders are under water, air trapped in the body hairs makes them appear silver (Fig. 1a).

In common with other spider species, the sexes are indistinguishable until they are subadult. As well as clubbed palps, the mature males have a broader, slightly shiny carapace, narrower abdomen and longer legs than the females (Fig. 1b). Their overall body length is usually several millimeters less than that of the females. Adult females are most conspicuous and unmistakable when they are carrying their bulky egg sacs: these vary in diameter from ca 8-15mm and are held in the chelicerae and often attached to the spinners as well. The abdomen is curled around the sac and so often only the carapace is visible (Fig. 1c).

   
Fig. 1a  D. plantarius underwater  Fig 1b Adult male D. fimbriatus Fig. 1c Adult female D. plantarius with egg sac

Is it Dolomedes plantarius or D. fimbriatus?

Definitive separation of the two European Dolomedes species used to rely on expert examination of microscopic characters that are only fully developed in adults; the species could not be distinguished as immatures. Adult female Dolomedes plantarius lack light hairs that, in D. fimbriatus, obscure the epigynal structures (Fig. 2a-e cf 2f). Even within D. plantarius there appears to be variation in the appearance of the epigyne. All of the images below (Fig. 2 a-d) have been sent to me identifed as D. plantarius  (please contact me if you doubt any of these identifications).

     
Fig.2a Epigyne of adult female D. plantarius from Germany. Click on the image for a larger version. Fig. 2b Epigyne of adult female D. plantarius from Sweden
Fig.2c Epigyne of adult female D. plantarius from Sweden. Click on the image for a larger version.
     awaiting image
 Fig.2d Dissection of epigyne from Italy (near Turin). Click on the image for a larger version. Fig. 2e  Dissection of epigyne of D. plantarius from Germany Fig. 2f Epigyne of adult female D. fimbriatus

 

The palps of adult male D. plantarius have a bifid tibial apophosis whereas those of D. fimbriatus have a simple structure (Fig. 3a cf b).

 

awaiting image

Fig. 3a Bifid apophysis on tibia of male D.plantarius palp

Fig. 3b Simple apophysis on tibia of male palp of D. fimbriatus

 Although it can never provide definitive identification, the two species tend to differ in their colouration. In the UK, colour variation, particularly of the body, tends to be greater in D. fimbriatus than in D. plantarius and the stripe width also tends to be greater (Fig. 4a). D. fimbriatus usually has a more prominent cardiac mark on the top of the abdomen than D. plantarius (Fig. 4b). Unbanded morphs occur at frequencies of up to 25% in all D. plantarius populations described to-date  but are very rare in D. fimbriatus. Figure 4c shows an unusual colour variant of D. fimbriatus from the New Forest, Hampshire, UK, in which the bands are very feint. Colour changes have been reported between moults although whether or not D. plantarius is banded is genetically determined. Newly-moulted juveniles often have a greenish tinge to their legs; this is particularly prominent in D. fimbriatus.

     
Fig. 4a  D. fimbriatus showing wide bands and conspicuous, light cardiac mark on abdomen Fig. 4b  Unbanded morph of D. plantarius Fig. 4c Unusual morph of D. fimbriatus with very faint band
 
Fig. 4d  Orange colour morph of D. plantarius from Italy Fig. 4e  Freshly-moulted, small juvenile D. fimbriatus
Fig. 4f  Adult female (right) and male (left) D. fimbriatus. Both have conspicuously lighter cardiac marks on their abdomens

Across Europe a much greater range of colour variation is encountered than in the UK. In Italy, D. plantarius can be much more orange in colour (Fig. 4d).

Sexual dimorphism tends to be greater in adult D. fimbriatus (Fig. 4f) than in D. plantarius but, again, this is not a definitive feature. Habitat type is also an indicator of species identity but the two species can occur in quite close proximity. In general D. fimbriatus is a species of acidic, wet heaths and upland mires. It is less dependent on open water than D. plantarius, which is a species of lowland fens, grazing marsh ditches, canals and other unpolluted neutral/alkaline water bodies.

In recent years, DNA fingerprinting has provided an alternative method of separating the two species. Although it is a specialist technique, and obtaining an identification is both slower and more expensive than by morphological examination, it has two very important advantages. Firstly, it and can be used to identify all developmental stages. Second, it can be used on small tissue samples (such as single legs) and even on the skins that are shed as the spiders grow (exuviae). This means that samples can be taken for identification without any damage to small and fragile populations.

Species that cause confusion

The most common source of confusion over the identity of Dolomedes species in the UK is with the semi-aquatic genus of Lycosid (or wolf) spiders, the Pirata species, several of which have more or less distinct white lateral lines on the carapace (Fig. 5a&b). Mature Piratas are similar in size to third or half-grown Dolomedes but their overall appearance is very different, with a high and squared-off front to the carapace and rather bulbous abdomen. In the breeding season, female Piratas are often seen carrying egg sacs but in contrast to Dolomedes, these are small (ca 5mm), white or bluish in colour and are carried at the back of the abdomen, in the spinners (Fig. 5b). When the eggs hatch, the tiny spiderlings are carried on the mother's back for about a week, until they can fend for themselves (Fig. 5c).

Fig. 5a  Pirata sp. showing white lateral lateral bands

Fig. 5b  Pirata sp. carrying egg sac

Fig. 5c  Pirata sp. carrying newly-hatched spiderlings on her back

Because it shares a similar habitat, the water spider Argyroneta aquatica is another source of confusion with Dolomedes. This is a truely aquatic species, living underwater and constructing silk retreats that are filled with air bubbles, making them resemble diving bells. They often occur in the same water bodies as D. plantarius although their appearance is very different.

The nursery web spider Pisaura mirabilis, is very closely related to the Dolomedes species and can look quite similar, although it occupies a very different habitat - it is a common species of grassland and tall herb vegetation. Confusion is most likely to arise where it is found in tall, water-side vegetation. Although smaller than Dolomedes (adult females are 12-15mm in length), P. mirabilis are a similar shape and very variable in colour, sometimes with bold abdominal markings and pale lateral lines (as in Fig. 6a). However, they resemble Dolomedes most closely when they are carrying their egg sacs and attending their nursery webs. As in Dolomedes, egg sacs are carried under the body, gripped by the chelicerae (Fig. 6b). The nursery webs, although smaller, are also very similar to those of Dolomedes (Fig. 6c).

Fig. 6a Pisaura mirabilis - colour variant with marked lateral stripes

Fig. 6b Pisaura mirabilis carrying egg sac

Fig. 6c Pisaura mirabilis with nursery web

 

To download a pocket ID card to help you to identify Dolomedes species in the field, click here. This card was produced by the Broads Authority.