OK, now I see something. The directory for the figures seems to be

not be

<http://www.discoverlife.org/20/>

but rather 

<http://www.discoverlife.org/20/q?see=I_JEL>


because each figure has a unique identifer after that JEL, such as 

952&res=md

or 

956&res=md


Maybe if I just use the main text link and 


<http://www.discoverlife.org/20/q?see=I_JEL>

It will work? Will HTtrack recognize it as a directory without a / or . or
something?