It is a true example of mind control in nature, and though scientists are well aware of the method of infection, they are uncertain exactly how the mind control is accomplished. When a wasp successfully attacks a host spider, the spider is temporarily paralyzed as the wasp lays eggs on the tip of the spider's abdomen. Once the wasp departs, the spider regains its ability to move, and it continues its daily web construction for the next two weeks as though nothing has changed. Meanwhile, the wasp's growing larvae cling to the spider's belly and feed on its juices through a number of small punctures.
This behavior was first observed by Dr. William G. Eberhard at the university of Costa Rica. His observations have led him to believe that the mind control is most likely accomplished through a fast-acting chemical secreted by the larvae, but what that chemical is-- and how it works-- is a mystery. What he has found is that the spider's usual five-step web building process is reduced to two when held captive by these larvae, resulting in the alternate design; and he has also discovered that if he removes the larvae on the last day, just before the spider is killed, the spider will often recover after a few days of spinning the abnormal web.
It is true that many parasites are able to shape their host's behavior subtly, but never before has science observed a parasite that can manipulate its host in such a detailed, specific way. As evidenced by this finding, biology certainly has many fascinating secrets yet to be discovered.