Corixidae Family - ID Tips
Class: Insecta; Order: Hemiptera; Suborder: Heteroptera; Family: Corixidae
|
Vital Stats |
Rating |
Scale |
|
BMWP Score |
5/10 |
1 = Filthy to 10 = pristine |
|
LIFE Flow Group |
4/6 |
1= Rapid velocity (>100cm/sec) to 6= drought resistant |
|
Family Level Difficulty Rating |
2/10 |
1= Easy to 10= very difficult |
|
Species Level Difficulty Rating |
6/10 |
1= Easy to 10= very difficult |
The British Corixidae (Lesser Water Boatmen) incorporate the subfamilies Corixinae, Cymatiainae and Micronectinae. They are typically encountered in lakes and ponds but may occasionally also be found in canals and slow moving sections of rivers; they are always absent from rapid streams. They are quite tolerant to moderately polluted waters. Attention is focussed here on the Corixinae which contains most of the species in the family and concentrates on some areas where difficulty may be encountered by less experienced workers using the FBA key for this group.
Identification
The Corixidae differ from other sub aquatic Hemiptera (bugs) by the lack of elongated piercing-sucking mouthparts; the head is of a characteristic triangular shape in anterior view.
As the males have more diagnostic features than the females, the sexes should be separated and the males identified first; females can often be matched up afterwards on the basis of the hemielytra patterning. The split is fairly straightforward on examination of the abdominal segments, the margins of which are parallel in females but very irregular in males.
Using the FBA key for the British Aquatic Hemiptera a distinction must be made in the first couplet for Corixinae on whether the hemielytra is smooth and shiny or wrinkled. As mentioned in the key this distinction can be difficult for beginners and it is made easier if the surface is dried although with practice this isn’t necessary.
Couplet 12 in the FBA key asks to differentiate on the basis of the appearance of the light lines on the corium. Going with narrow/mainly unbroken takes you to the genus of Hesperocorixa, however, the key mentions that confusion may occur with specimens of Sigara falleni and S.distincta which also have relatively narrow transverse lines; examination of figures 4 and 5 still show a clear difference however.
At couplet 22 we have a group of three similar looking species, the arrangement of pegs on the pala can be used to distinguish males and the shape of the pronotum is useful for both sexes. Sigara fallenoidea has only been recorded from Ireland within the British Isles.
Females can be distinguished from the similar looking Sigara dorsalis by counting the number of small spines on the dorsal surface of the posterior femur (only 3-6 in S.dorsalis)
An additional species Sigara iactans, is not mentioned in the FBA key being a probable recent coloniser from continental Europe. It has now been found in various locations in SE England. It often occurs with and is very similar to S. falleni and is best separated by means of the male palae. The pala of S. falleni is somewhat triangular, being widest at the base; the inner and distal rows of pegs would meet if continued by eye. The pala of S. iactans is trapezoidal in shape with the widest part being nearer the outer end; the two rows of pegs would not meet if continued by eye.
References
Brooke, S. Het News, 2nd Series, no.5, Spring 2005
Savage, A.A.(1989) Adults of the British aquatic Hemiptera Heteroptera. a key with ecological notes. Freshwater Biological Association. Scientific Publication 50
