• Category
  • Length Range
    84-162 µm
  • Width Range
    12-18 µm
  • Striae in 10 µm
  • Synonyms
    Pinnularia nodosa var. formica (Ehrenb.) Cleve



Valves with a large expanded central valve and distinctly capitate apices. The center expansion of the valve is longitudinally elliptic. The striae are constrained to the margin of the valve face, creating a large axial area, up to roughly 2/3 of the valve face. Striae are weakly radiate to parallel near the central area, transitioning to convergent at the apices. Striae are longer near the apices. The raphe is lateral. Proximal raphe ends are deflected unilaterally and terminate in a teardrop shape. Within a frustule, the raphe of each valve are offset to one another. The distal ends of the raphe are strongly deflected toward the same side. Distal ends of the raphe terminate in a "question mark" shape.


Pinnularia formica has been found in lentic and lotic freshwater systems along the east coast of the US. The specimens shown here were found in surface sediments of lakes in the northeast.

P  Formica Biodata Map
Credit: USGS Biodata
Pinnularia formica occurence in river surveys by USGS.
Credit: www.CT.gov - Connecticut Department of Energy and Environmental Protection.
Halls Pond, Maine - a site where P. formica has been reprted.

Original Description

144 N. formica, testula a dorso anguste linearies quater constricta, articulis spuriis oblongis. Maine A, nec Bridgwater.

  • Basionym
    Navicula formica
  • Author
    Ehrenb. 1843

Original Images

Screen  Shot 2018 01 30 At 4 11 20  Pm

Cite This Page

Tyree, M., Ripple, H. (2018). Pinnularia formica. In Diatoms of North America. Retrieved August 11, 2020, from https://diatoms.org/species/pinnularia_formica


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Pinnularia formica from all the stream reaches where it was present. Note that the relative abundance scale is the same on each plot. Explanation of each environmental variable and units are as follows:

ELEVATION = stream reach elevation (meters)
STRAHLER = distribution plot of the Strahler Stream Order
SLOPE = stream reach gradient (degrees)
W1_HALL = an index that is a measure of streamside (riparian) human activity that ranges from 0 - 10, with a value of 0 indicating of minimal disturbance to a value of 10 indicating severe disturbance.
PHSTVL = pH measured in a sealed syringe sample (pH units)
log_COND = log concentration of specific conductivity (µS/cm)
log_PTL = log concentration of total phosphorus (µg/L)
log_NO3 = log concentration of nitrate (µeq/L)
log_DOC = log concentration of dissolved organic carbon (mg/L)
log_SIO2 = log concentration of silicon (mg/L)
log_NA = log concentration of sodium (µeq/L)
log_HCO3 = log concentration of the bicarbonate ion (µeq/L)
EMBED = percent of the stream substrate that is embedded by sand and fine sediment
log_TURBIDITY = log of turbidity, a measure of cloudiness of water, in nephelometric turbidity units (NTU).
DISTOT = an index of total human disturbance in the watershed that ranges from 1 - 100, with a value of 0 indicating of minimal disturbance to a value of 100 indicating severe disturbance.