genus Catocala (Underwing Moths)
Up to family Noctuidae

Habilis Underwing
Habilis Underwing (Catocala habilis, Hodges # 8778)
Fermilab, Kane County, Illinois
13 October 2005

The front wings of underwing moths often look very similar among different species.  So in order to identify them, I either coax the moth to open its wings by disturbing it, or gently lift one front wing, as you see in these photos.  The hind wing can provide some key identification features.

Note the dates of these photos.  Underwings are mostly late summer to autumn moths, although some smaller ones seem to have two generations per summer. 

All of these are about 2.5 to 3.5 inch wingspans. 
Obscure Underwing Obscure Underwing (Catocala obscura, Hodges # 8784)
Fermilab, Kane County, Illinois
17 August 2009

This is the only underwing with black hind wings that I have found here so far in the Chicago area. 
The Bride Underwing
The Bride Underwing (Catocala neogama, Hodges # 8798)
Fermilab, Kane County, Illinois
11 September 2009 

Yellow-banded Underwing (Catocala cerogama, Hodges # 8802)
Fermilab, Kane County, Illinois
1 September 2006

For the several years after this photo was taken (during 2007 and 2008), the yellow-banded underwing was the one that I most often encountered at Fermilab. 
Once-married Underwing (Catocala unijuga, Hodges # 8805)
Fermilab, Kane County, Illinois
1 September 2006
Mother Underwing
Mother Underwing (Catocala parta, Hodges # 8806)
Fermilab, Kane County, Illinois
2 September 2009

This is a fairly common underwing in late summer and early fall at Fermilab.  According to "A Field Guide to Moths of Eastern North America," by Charles V. Covell, Jr., a key identification feature is the presence of the dark rays on the forewing near the apex, near the body and parallel to the trailing edge. 
Bronze Underwing (Catocala cara, Hodges # 8832)
Fermilab, Kane County, Illinois
13 September 2006

This strikingly beautiful underwing moth has a unique purplish tint on the front wings.  It is common at Fermilab.