(276FOsex) Folsomia sexoculata (Tullberg, 1871)

*britannica Stach, 1947
*folsomiella (Bagnall 1949)

Folsomia sexoculata is a common species which is usually found in the margins of tidal rivers and estuaries (e.g. Kew), but there are also a few inland records from the margins of freshwater (such as Derwentwater in the Lake District and Harewood Bridge in Yorkshire).  It is possible that these could represent another, undescribed, 3-eyed Folsomia species, but will remain with the brackish-zone records until taxonomic evidence emerges to the contrary.
Folsomia sexoculata is blackish-grey and has 3+3 ocelli (2 close together near the PAO and one clearly separate on each side of the head; Figs. 1, 2). It reaches a maximum length of 2.5 mm. The body setae are short; the ratio of the length of the longest setae at the tip of the abdomen/length of mucro is between 2.0 and 2.5.  Steve Hopkin observed variability in the  manubrial setae, with 2+1, 2+2, 2+3 or 3+3 apical ventral (anterior) setae (e.g. Fig. 3) all occurring. The dens has 13-17 ventral (anterior) setae and 6 dorsal (posterior) setae (4 at the base of the dens and 2 in the mid region). The post-antennal organ (PAO) is narrow and is shorter than the width of ant1 (Fig. 1).


The main distinction from Folsomia thalassophila is the ratio of the length of the longest setae at the tip of the abdomen/length of mucro. In Folsomia sexoculata this is never more than 2.5 whereas in Folsomia thalassophila the ratio is between 3.0 and 3.8.  Steve notes having seen mature males and females of both species and being confident that the separation was safe despite the poor condition of many of the specimens of these in the NHML collection.


 

 

Fig. 1: Folsomia sexoculata collected from a 'riparian' site in Harewood, Yorks by Bagnall (date not known). There are two anterior ocelli set close together near the post-antennal organ (PAO) and a single posterior ocellus.

Fig. 2: Folsomia sexoculata collected from the tidal zone of the River Thames at Queen's Head steps, London in March 1971 by H.F. Shearman. There are two anterior ocelli set close together near the PAO. The single posterior ocellus is out of the plane of focus.

Fig. 3: Same specimen of Folsomia sexoculata shown in Fig. 1. The manubrium (ma) has 3+2 ventral (anterior) setae (*). d, dens.

 
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