<li><a href="<?php echo get_option('home'); ?>/" class="on"><span>Home</span></a></li>

<?php
echo preg_replace('@\<li([^>]*)>\<a([^>]*)>(.*?)\<\/a>@i', '<li$1><a$2><span>$3</span></a>', wp_list_categories('show_count=0&echo=0&exlude=181&title_li='));
?>