Help with rewrite rules

I have a .htaccess in my root web dir with rewrite rules already setup, something like:
Options -Indexes
RewriteEngine on
RewriteBase /
RewriteCond %{HTTP_HOST} !^www\.mydomain\.com\[NC]
RewriteRule ^([^/]*)\.html$ $1.php?%{QUERY_STRING} [NC]
....
.....

Firstly is it possible to add rewrite rules for subdirectories in this root webdir .htaccess?

What I want to do is add rewrite calls for any files / subdirectories in a specific subdirectory to be rewritten to the main index.html page - and do this ONLY WHEN a specific remote host attempts to access them.
So for example, the remote host is called crawler.remotehost.com
and the subdirectory which I need anything under (including just it) to be rewritten is:
/subdir
rewritten to
index.html

so for example, when crawler.remotehost.com visits
www.mydomain.com/subdir
it is sent to
www.mydomain.com/index.html

or when crawler.remotehost.com visits
www.mydomain.com/subdir/1.php
it is sent to
www.mydomain.com/index.html

or when crawler.remotehost.com visits
www.mydomain.com/subdir/subsubdir
it is sent to
www.mydomain.com/index.html

or when crawler.remotehost.com visits
www.mydomain.com/subdir/subsubdir/2.php
it is sent to
www.mydomain.com/index.html

Hope this makes sense

 

RewriteCond %{HTTP_HOST} ^crawler\.remotehost\.com$
RewriteCond %{REQUEST_URI} ^subdir
RewriteRule ^(.*)$ /index.html [R=301,L]

Code (markup):

will rewrite anything beginning with "subdir" to /index.html, but only if the host matches crawler.remotehost.com

 

Hi again John

Thanks for your help, when I add (for testing) the below lines to (the the front or end) of my existing .htaccess file (see my last post for extract of it) in the web root directory :
RewriteCond %{REQUEST_URI} ^subdir
RewriteRule ^(.*)$ /index.html [R=301,L]

Nothing happens (ie. it doesn't redirect me to index.html) when I go to "subdir" or any page / subdirectory under it...

Do these lines have to go anywhere in particular in my existing .htaccess, or do they have to sit in another .htaccess undder "subdir", or something else?

Once I get this to work, I will add the first line (RewriteCond %{HTTP_HOST} ^crawler\.remotehost\.com$) and test it (one step at a time):

 

Thanks John, got it working, I had to change:
RewriteCond %{REQUEST_URI} ^subdir
to
RewriteCond %{REQUEST_URI} ^/subdir/

to get it to work

Thanks mate.