(298HApos) Halisotoma poseidonis (Bagnall, 1939)

Halisotoma poseidonis (formerly Isotoma poseidonis) is a scarce inhabitant of the seashore, although it has probably been confused with Halisotoma maritima in the past and may be more common than records would indicate. Halisotoma poseidonis is greyish blue in colour and reaches a length of 2.5 mm. The mucro has only three teeth and the outer side of the tibiotarsus of the second leg bears a pair of distinctive fan setae (Fig. 1). Halisotoma poseidonis differs from Halisotoma maritima principally in the fine structure of the maxilla (Fig. 2), but also has a stouter claw with larger lateral spines, but a thinner seta A7 adjacent to the claw.

 

 

 

Fig. 1: Halisotoma poseidonis collected from Cramond shore, Scotland in March 1937 by Bagnall. The outer side of the tibiotarsus of the second leg bears a pair of distinctive fan setae. The asterisks (*) mark the points of insertion of the fan setae.

Fig. 2: Maxilla (max) and mandible (man) of the same specimen of Halisotoma poseidonis shown in Fig. 1. Note that lamella 1 of the maxilla (max) has two 'ciliated' fringes (lamella 1 of Halisotoma maritima has about five 'ciliated' fringes).

 
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