Morning glories often loose the older leaves near the base of the plant to relocate nutrients to the part of the plant that is most actively growing the most >which is the stem tips and any flowering structures...

The older leaves turning yellow and falling off is normal and unrelated to any large spiders on the plant...true spiders are carnivorous and eat insects like herbivores,so you might have even less leaves if the spiders weren't eating any leaf eating insects...

Despite the widespread and quasi-pernicious MYTHS about nutrients and Morning Glories,the lower leaves can sometimes be prevented from falling off by the application of some bloom booster fertilizer...something with an NPK ratio of about 10-30-10...

The word fertilizer is a very fuzzy term that may include a large number of different compounds and elements with very different effects...

Nitrogen in excessive amounts may initially delay flowering,but once flowering has started the nitrogen will not impair the flowering...phosphorus is bloom stimulating and the potassium will help in building a strong immune system...

Here are some links to where I have shared on the subject of nutrients previously...

http://forums.gardenweb.com/forums/load/vines/msg0317360012541.html
http://forums.gardenweb.com/forums/load/vines/msg0522311832703.html
http://forums.gardenweb.com/forums/load/vines/msg080802299604.html
http://forums.gardenweb.com/forums/load/vines/msg080802299604.html
http://forums.gardenweb.com/forums/load/vines/msg040710098176.html

Hope that helps...

Talk to you...

Ron

Here is a link that might be useful: Morning Glories and fertilizer...myths and reality...