special resin secreeted from some sort of glands on their body in order to capture and hold hosts. the aliens contain more than one strand of DNA and without the presence of a queen can actually create a single egg bearing a 'super facehugger'
this theory is based on the events on the nostromo (see alien directors cut) where ripley discovers bret. he has undergone almost a complete transformation bearing resembalence to an single alien egg.
this is possible from DNA toxins induced into the human from the piercing of the flesh with the alien tail, the chemicals completely change the biological structure of human dna and render the human change to an alien egg.
the presence of dallas cocooned was not for the same reason. he was to be host material for the alien offspring. ripleys records state that within almost 24 hours the alien managed to wipe out the entire crew of the nostromo, this gives us some insight into the length of time for this particular type of lifecycle.