HR Giger:The Hand of Alien Monster IV

 leading from

   HR Giger: Alien Monster IV

a) Crane Fly Legs
The hand of Alien Monster has long fingers almost like tendrils. It seems part inspired by the Face Hugger and perhaps inspired by the fragile legs of a crane fly

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Daddy Longlegs spider

b) Hidden hand idea buried in Treasures of Satan
In The painting Treasures of Satan, approximately where the hand ought to be, there are fingery tendrils of fire and a dark spike of shadow that might have been mentally transfomed into a hand by Giger's mind . However the hand imagined there does not seem to be facing the right direction.

Perhaps the tendrils of fire became fingers and a vertical shadow became the thumb

c) Hand takes on an impression of the body of the doomed soul.
The body of a doomed soul in the painting might also have served as inspiration to give form to the hand and indeed the hand has been turned to complete the impression of looking as if it is mimicking the form of the body of the doomed soul

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Perhaps the hand was also inspired by the general outline of the body of the doomed soul

  

Jean Delville's Treasures of Satan