[2] They are found only at the top of the shore, around the High-Water Spring drift line on the seaward side of the shingle bank, but this may well be due to the unstable nature of this part of the beach, and the fact that the fauna is usually sparse.

The animal is found far lower down on the landward side where it feeds on Sphaeroma and is liable to spasmodic immersion at high spring tides and during bad weather.

Immersed animals migrate up the beach suggesting that even the adults are not well adapted to resist fully marine conditions for long.

[3] Mature females migrate into moist sandy areas where conditions approximate to terrestrial ones to lay their eggs, and to brood their young which are rapidly desiccated in unsaturated air.

This gearing is reflected in the size of the oocytes in the ovary, which varies only very slightly between different individuals at a given time of the year.

Such migrations suggest that the spermatophores, eggs, larval stages and moulting animals are unable to withstand much immersion in sea water.

Although Strigamia was almost completely absent from the salt marsh proper, a large number of individuals were frequently found at its edge in a narrow belt a few feet wide.

The extremely high densities of Strigamia maritima seen on North European shores, specially in Scotland, are very striking (Figure 1).

Strigamia maritima was also recorded from the coasts of Scandinavia, Germany, Netherlands, Belgium, Britain, Ireland and northern France.

Bergesen et al. (2006) recorded it for the first time for North Norway (under stones in the supralittoral amongst isopods, Porcellio scaber).

The life history and ecology of this species was studied by Lewis (1961)[6] who related its behavior and its tolerance of seawater at different stages of its life history to it being a mobile species, concentrating in areas that are climatically favorable, have a good food supply and to its breeding season also its ability to migrate up and down beaches.

The vas deferens divides two thirds of the way along its length to pass round the gut, and fuses again before opening ventrally on a sub-terminal penis.

[7] Two pairs of tubular accessory glands lie alongside the gut and open posteriorly into the genital atrium.

long and gradually become rolled up in the distal region of the vas deferens, and lower down are coiled on themselves in the manner of a clock spring.

[5] The anatomy of the female reproductive system: S. maritima resembles closely that described for other genera by Fabre (1855) and Schaufler (1889), consisting of an unpaired tubular ovary leading to a short oviduct which divides to pass round the gut, fusing again to open ventrally into the subterminal genital atrium.

the egg shell splits equatorially, and as the embryo, which is bent into a horseshoe shape, gradually elongates and uncurls during development.

In S. maritima hatching begins at what Verhoeff terms the last embryonic stage, in which the limbs and mouthparts are represented by simple buds, and the body, covered with cuticle, is inflated anteriorly since it still contains a large quantity of yolk.

[8] As the egg teeth, helped by the increase of yolk in the front part, break the egg-shell the last embryonic stadium appears.

[6] In this stadium, most characters of the mature animal are apparent, but the numbers of seta, sensory sensilla and coxal glands are increasing through the stadia to follow.

When attacking larger Orchestia, 1 cm or more in length (which it only did if they were damaged or dying) it made a transverse slit and pushed its head and anterior segments inside and the poison claws were seen to be constantly in motion macerating tissue whilst the centipede was involved in what seemed to be external digestion and suctorial feeding.

[6] In the naturally occurring variation in the segment number of Strigamia maritima, it is clear that the developmental process, as described above, is being influenced either by genetic or environmental factors, or possibly by both.

However, if the explanation was solely of this latter kind, i.e. all due to direct environmental effects and no selective component, then it is hard to explain why the observed pattern of between-species variation mirrors its intraspecific equivalent, with more northerly species having fewer segments than their southern counterparts.

[12] One compound hypothesis is that speciating peripheral isolates at northern or southern ends of a parent species’ range go through a phase of having a restricted amount of variation in segment number.